spi: fsl-espi: simplify fsl_espi_setup_transfer

If t is not null then the SPI core takes care that bits_per_word and
speed_hz are populated. This allows to simplify fsl_espi_setup_transfer.

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