iio: imu: st_lsm6dsx: check if master_enable is located in primary page

Check if the master enable register is located in the primary or in the
secondary memory page. This is a preliminary patch to support i2c master
controller on lsm6dsm devices

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>