iio: mma9551: fix sparse warning

sparse warnings: (new ones prefixed by >>)

>> drivers/iio/accel/mma9551.c:554:57: sparse: mixing different enum types
   drivers/iio/accel/mma9551.c:554:57:     int enum mma9551_tilt_axis  versus
   drivers/iio/accel/mma9551.c:554:57:     int enum mma9551_gpio_pin
>> drivers/iio/accel/mma9551.c:576:57: sparse: mixing different enum types
   drivers/iio/accel/mma9551.c:576:57:     int enum mma9551_tilt_axis  versus
   drivers/iio/accel/mma9551.c:576:57:     int enum mma9551_gpio_pin

Signed-off-by: Vlad Dogaru <vlad.dogaru@intel.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Reviewed-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>