spi/spi-fsl-spi: Move setting non-zero tx and rx shifts to a function accessed...

spi/spi-fsl-spi: Move setting non-zero tx and rx shifts to a function accessed by a function pointer

Acked-by: Anton Vorontsov <anton@enomsg.org>
Signed-off-by: Andreas Larsson <andreas@gaisler.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>