spi: fsl-dspi: remove useless code for dspi driver.

Since we are using regmap framework's internal locks, so the
lock_arg for dspi_regmap_config is redundant here.

This patch just remove it, and then the dspi_regmap_config could
be const type.

Signed-off-by: Xiubo Li <Li.Xiubo@freescale.com>
Signed-off-by: Mark Brown <broonie@kernel.org>