spi: spi-fsl-dspi: Rename fifo_{read,write} and {tx,cmd}_fifo_write

These function names are very generic and it is easy to get confused.
Rename them after the hardware register that they are accessing.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20200304220044.11193-6-olteanv@gmail.com


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