mpc8xxx_spi: always use 8-bit characters, don't read or write garbage

mpc8xxx_spi: always use 8-bit characters, don't read or write garbage

There are a few problems with the current driver.

First, it unconditionally reads from dout/writes to din whether or not
those pointers are NULL. So for example a simple "sf probe" ends up
writing four bytes at address 0:

=> md.l 0x0 8
00000000: 45454545 45454545 05050505 05050505    EEEEEEEE........
00000010: 00000000 00000000 07070707 07070707    ................
=> sf probe 0
mpc8xxx_spi_xfer: slave spi@7000:0 dout 0FB53618 din 00000000 bitlen 8
mpc8xxx_spi_xfer: slave spi@7000:0 dout 00000000 din 0FB536B8 bitlen 48
SF: Detected s25sl032p with page size 256 Bytes, erase size 64 KiB, total 4 MiB
=> md.l 0x0 8
00000000: ff000000 45454545 05050505 05050505    ....EEEE........
00000010: 00000000 00000000 07070707 07070707    ................

(here I've change the first debug statement to a printf, and made it
print the din/dout pointers rather than the uints they point at).

Second, as we can also see above, it always writes a full 32 bits,
even if a smaller amount was requested. So for example

=> mw.l $loadaddr 0xaabbccdd 8
=> md.l $loadaddr 8
02000000: aabbccdd aabbccdd aabbccdd aabbccdd    ................
02000010: aabbccdd aabbccdd aabbccdd aabbccdd    ................
=> sf read $loadaddr 0x400 6
device 0 offset 0x400, size 0x6
mpc8xxx_spi_xfer: slave spi@7000:0 dout 0FB536E8 din 00000000 bitlen 40
mpc8xxx_spi_xfer: slave spi@7000:0 dout 00000000 din 02000000 bitlen 48
SF: 6 bytes @ 0x400 Read: OK
=> sf read 0x02000010 0x400 8
device 0 offset 0x400, size 0x8
mpc8xxx_spi_xfer: slave spi@7000:0 dout 0FB53848 din 00000000 bitlen 40
mpc8xxx_spi_xfer: slave spi@7000:0 dout 00000000 din 02000010 bitlen 64
SF: 8 bytes @ 0x400 Read: OK
=> md.l $loadaddr 8
02000000: 45454545 45450000 aabbccdd aabbccdd    EEEEEE..........
02000010: 45454545 45454545 aabbccdd aabbccdd    EEEEEEEE........

Finally, when the bitlen is 24 mod 32 (e.g. requesting to read 3 or 7
bytes), the last three bytes and up being the wrong ones, since the
driver does a full 32 bit read and then shifts the wrong byte out:

=> mw.l $loadaddr 0xaabbccdd 4
=> md.l $loadaddr 4
02000000: aabbccdd aabbccdd aabbccdd aabbccdd    ................
=> sf read $loadaddr 0x444 10
device 0 offset 0x444, size 0x10
mpc8xxx_spi_xfer: slave spi@7000:0 dout 0FB536E8 din 00000000 bitlen 40
mpc8xxx_spi_xfer: slave spi@7000:0 dout 00000000 din 02000000 bitlen 128
SF: 16 bytes @ 0x444 Read: OK
=> md.l $loadaddr 4
02000000: 552d426f 6f742032 3031392e 30342d30    U-Boot 2019.04-0
=> mw.l $loadaddr 0xaabbccdd 4
=> sf read $loadaddr 0x444 0xb
device 0 offset 0x444, size 0xb
mpc8xxx_spi_xfer: slave spi@7000:0 dout 0FB536E8 din 00000000 bitlen 40
mpc8xxx_spi_xfer: slave spi@7000:0 dout 00000000 din 02000000 bitlen 88
SF: 11 bytes @ 0x444 Read: OK
=> md.l $loadaddr 4
02000000: 552d426f 6f742032 31392e00 aabbccdd    U-Boot 219......

Fix all of that by always using a character size of 8, and reject
transfers that are not a whole number of bytes. While it ends being
more work for the CPU, we're mostly bounded by the speed of the SPI
bus, and we avoid writing to the mode register in every loop.

Based on work by Klaus H. Sørensen.

Cc: Klaus H. Sorensen <khso@prevas.dk>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>