iio:magnetometer:bmc150_magn: use descriptive name for mask

Define and use a descriptive name for the repetition registers data mask,
instead of a 'magic' value.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>