spi: fsl-espi: simplify and inline function fsl_espi_change_mode

The ESPI spec mentions no requirement to turn off the ESPI unit prior
to changing the mode. Most likely the ESPI unit is only turned off to
clear the FIFO's as before this patch series single bytes could
remain in the TX FIFO after transfer end.
Therefore remove disabling / re-enabling the ESPI unit.

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