spi: spi-fsl-dspi: Use BIT() and GENMASK() macros

Switch to using more idiomatic register field definitions, which makes
it easier to look them up in the datasheet. Cosmetic patch.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20190818180115.31114-4-olteanv@gmail.com


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