iio:magnetometer: bugfix magnetometers gain values

commit a31d0928999fbf33b3a6042e8bcb7b7f7e07d094 upstream.

This patch fix gains values. The first driver was designed using
engineering samples, in mass production the values are changed.

Signed-off-by: Denis Ciocca <denis.ciocca@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>