iio: st_sensors: Return as early as possible from the _write_raw() callbacks

As there is no cleanup to do, let's return as early as possible in the
various ST sensor drivers _write_raw() callback functions.

There is no functional change.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Cc: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20220207143840.707510-5-miquel.raynal@bootlin.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>