mfd: wm31x: Fix Wvoid-pointer-to-enum-cast warning

mfd: wm31x: Fix Wvoid-pointer-to-enum-cast warning

'type' is an enum, thus cast of pointer on 64-bit compile test with
W=1 causes:

  wm831x-spi.c:36:10: error: cast to smaller integer type 'enum wm831x_parent' from 'const void *' [-Werror,-Wvoid-pointer-to-enum-cast]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230810095849.123321-8-krzysztof.kozlowski@linaro.org
Signed-off-by: Lee Jones <lee@kernel.org>