staging: iio: hmc5843_core: fix sparse warnings

Fix the following sparse warnings:

 CHECK   drivers/staging/iio/magnetometer/hmc5843_core.c
drivers/staging/iio/magnetometer/hmc5843_core.c:138:70: warning:
incorrect type in argument 3 (different signedness)
drivers/staging/iio/magnetometer/hmc5843_core.c:138:70:    expected
unsigned int *val
drivers/staging/iio/magnetometer/hmc5843_core.c:138:70:    got int
*<noident>
drivers/staging/iio/magnetometer/hmc5843_core.c:215:64: warning:
incorrect type in argument 3 (different signedness)
drivers/staging/iio/magnetometer/hmc5843_core.c:215:64:    expected
unsigned int *val
drivers/staging/iio/magnetometer/hmc5843_core.c:215:64:    got int
*<noident>
drivers/staging/iio/magnetometer/hmc5843_core.c:354:72: warning:
incorrect type in argument 3 (different signedness)
drivers/staging/iio/magnetometer/hmc5843_core.c:354:72:    expected
unsigned int *val
drivers/staging/iio/magnetometer/hmc5843_core.c:354:72:    got int
*<noident>
drivers/staging/iio/magnetometer/hmc5843_core.c:362:72: warning:
incorrect type in argument 3 (different signedness)
drivers/staging/iio/magnetometer/hmc5843_core.c:362:72:    expected
unsigned int *val
drivers/staging/iio/magnetometer/hmc5843_core.c:362:72:    got int
*<noident>

Signed-off-by: Teodora Baluta <teodora.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>