sf: Simplify fastest read cmd code

sf: Simplify fastest read cmd code

Fastest read command code look for fastest read command
taking inputs from spi->mode_rx and flags from param table
and controller mode_rx is always been a priority.

Since mode_rx is always set from controller side this optimized
code doesn't require much and this code required exctra overhead like
1) Maintain e_rx_cmd in param table
2) Maintain mode_rx in spi_slave {}

Hence removed this code, and look for read command from normal
spi->mode from spi_slave{} and params->flags

Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Cc: Vignesh R <vigneshr@ti.com>
Cc: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Jagan Teki <jteki@openedev.com>