iio: st_sensors: Fix buffer copy

Use byte_for_channel as iterator to properly initialize the buffer.

Signed-off-by: Robin van der Gracht <robin@protonic.nl>
Acked-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: <Stable@vger.kernel.org>