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 <[email protected]>
Cc: Bin Meng <[email protected]>
Cc: Michal Simek <[email protected]>
Cc: Siva Durga Prasad Paladugu <[email protected]>
Cc: Vignesh R <[email protected]>
Cc: Mugunthan V N <[email protected]>
Signed-off-by: Jagan Teki <[email protected]>