net: macb: Fix rx buffer cache handling

net: macb: Fix rx buffer cache handling

With commit c6d07bf440bc ("net/macb: increase RX buffer size for GEM")
ethernet support does not work any more with d-cache enabled on the
AT91SAM. The reason is, that MACB_RX_BUFFER_SIZE was changed from 4096
to 128 but this change was not refected in the rx_buffer flush and
invalidate functions, as these also use this macro.

This patch now fixes this by calculating the rx buffer size correctly
again in those functions. With this change, ethernet works again
reliably on my AT91SAM board.

Signed-off-by: Stefan Roese <sr@denx.de>
Fixes: c6d07bf440bc ("net/macb: increase RX buffer size for GEM")
Cc: Ramon Fried <rfried.dev@gmail.com>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Cc: Anup Patel <anup.patel@wdc.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>