spi: spi-fsl-dspi: Don't mask off undefined bits

This is a useless operation, and if the driver needs to do that, there's
something deeply wrong going on.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20200304220044.11193-4-olteanv@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>