gpio: mxs: fix Wvoid-pointer-to-enum-cast warning

gpio: mxs: fix Wvoid-pointer-to-enum-cast warning

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

  gpio-mxs.c:274:16: error: cast to smaller integer type 'enum mxs_gpio_id' from 'const void *' [-Werror,-Wvoid-pointer-to-enum-cast]

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>