spi: spi-fsl-dspi: Remove duplicated register write

SPI core makes sure that transfer speed is always set so code here writes
the same register with the same value twice. Code has been doing this from
the beginning.

This looks to me some sort of copy paste error so I'm removing the second
write. If this is not the case we can bring it back with a comment.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>