spi: fsl-espi: merge fsl_espi_cmd_trans and fsl_espi_rw_trans

fsl_espi_cmd_trans and fsl_espi_rw_trans share most of the code so
we can merge them.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>