iio:accel:bmc150-accel: make use of mask definition

BMC150_ACCEL_SLOPE_THRES_MASK was defined some time ago, but its 'magic'
value got used instead in bmc150_accel_write_event(). Make use of it for
improved readability.
Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Reviewed-by: Octavian Purdila <octavian.purdila@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/accel/bmc150-accel.c

@@ -776,7 +776,7 @@ static int bmc150_accel_write_event(struct iio_dev *indio_dev,
 
 	switch (info) {
 	case IIO_EV_INFO_VALUE:
-		data->slope_thres = val & 0xFF;
+		data->slope_thres = val & BMC150_ACCEL_SLOPE_THRES_MASK;
 		break;
 	case IIO_EV_INFO_PERIOD:
 		data->slope_dur = val & BMC150_ACCEL_SLOPE_DUR_MASK;