iio: imu: st_lsm6dsx: do not use a fixed read len in read_oneshot

Generalize st_lsm6dsx_shub_read_oneshot in order to not use a fixed
read length and take into account iio channel realbits for single
read operations

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