regulator: da9121: Fix Wvoid-pointer-to-enum-cast warning

regulator: da9121: Fix Wvoid-pointer-to-enum-cast warning

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

  da9121-regulator.c:1132:24: error: cast to smaller integer type 'enum da9121_subvariant' from 'const void *' [-Werror,-Wvoid-pointer-to-enum-cast]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://patch.msgid.link/20250509142448.257199-2-krzysztof.kozlowski@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>