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>