regulator: ltc3589: Fix Wvoid-pointer-to-enum-cast warning

regulator: ltc3589: Fix Wvoid-pointer-to-enum-cast warning

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

  ltc3589.c:394:22: error: cast to smaller integer type 'enum ltc3589_variant' from 'const void *' [-Werror,-Wvoid-pointer-to-enum-cast]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230810111914.204847-1-krzysztof.kozlowski@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>