spi: sf: rework the logic of selecting byte/word program for sst

Now byte program routine is installed for the flash write op only
when SPI controller has SPI_OPM_TX_BP tx operation mode. Previous
logic will cause such flash like SST25VF064C wrongly using the
byte program as the write op.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>