spi: spi-fsl-dspi: Drop unneeded use of dataflags bits

Checking directly against pointer value should be at least as fast as doing
bitmasking and compare, so let's keep it simple.

Signed-off-by: Esben Haabendal <eha@deif.com>
Acked-by: Martin Hundebøll <martin@geanix.com>
Signed-off-by: Mark Brown <broonie@kernel.org>