spi: fsl-dspi: Remove unneeded cast to same type

There is never a need to cast a pointer to the same pointer type.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/a417735ca3ff629ee897327b163b23414673f0a3.1678704562.git.geert+renesas@glider.be


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