spi: spi-fsl-dspi: Add comments around dspi_pop_tx and dspi_push_rx functions

Their names are confusing, since dspi_pop_tx prepares a word to be
written to the PUSHR register, and dspi_push_rx gets a word from the
POPR register.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20200304220044.11193-5-olteanv@gmail.com


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