iio: imu: st_lsm6dsx: take into account ts samples in wm configuration

Take into account hw timer samples in pattern length computation done
in st_lsm6dsx_update_watermark routine for watermark configuration.
Moreover use samples in pattern (sip) already computed in
st_lsm6dsx_update_decimators routine

Fixes: 21345107 ("iio: imu: st_lsm6dsx: add hw timestamp support")
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>