mfd: rsmu_spi: Remove unneeded casts of void *

There is no need to cast a "void *" to a different type of pointer.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/ae84c1751e79cb49ab584557f4ecd835a8493d7c.1678704562.git.geert+renesas@glider.be