iio:magnetometer: st_magn: add BDU settings

LIS3MDL and LSM9DS1 are missing BDU settings in their register maps, so add
them. I don't have a LIS3MDL sensor to test, but this works correctly on
the LSM9DS1, which has the same register map.

Signed-off-by: Martin Kelly <martin@martingkelly.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>