spi: spi-fsl-dspi: Remove pointless assignment of master->transfer to NULL

Introduced in commit 9298bc72

 ("spi: spi-fsl-dspi: Remove
spi-bitbang") for less than obvious reasons, this assignment is
confusing and serves no purpose.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20190818180115.31114-9-olteanv@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>