iio: imu: st_lsm6dsx: introduce st_lsm6dsx_sensor_set_enable routine

Add st_lsm6dsx_sensor_set_enable routine and remove
st_lsm6dsx_sensor_{enable/disable} ones in order to make the code more
readable and remove unnecessary functions

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