iio: gyro: bmg160: use available_scan_masks

Use available_scan_masks to allow the iio core to select
the data to send to userspace depending on which axes are
enabled, instead of doing this in the driver's interrupt
handler.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>