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

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

stmpe-i2c.c:90:13: error: cast to smaller integer type 'enum stmpe_partnum' from 'const void *' [-Werror,-Wvoid-pointer-to-enum-cast]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230810095849.123321-1-krzysztof.kozlowski@linaro.org


Signed-off-by: Lee Jones <lee@kernel.org>