iio: imu: fxos8700: fix MAGN sensor scale and unit

iio: imu: fxos8700: fix MAGN sensor scale and unit

+/-1200uT is a MAGN sensor full measurement range. Magnetometer scale
is the magnetic sensitivity parameter. It is referenced as 0.1uT
according to datasheet and magnetometer channel unit is Gauss in
sysfs-bus-iio documentation. Gauss and uTesla unit conversion
relationship as follows: 0.1uT = 0.001Gs.

Set magnetometer scale and available magnetometer scale as fixed 0.001Gs.

Fixes: 84e5ddd5c46e ("iio: imu: Add support for the FXOS8700 IMU")
Signed-off-by: Carlos Song <carlos.song@nxp.com>
Link: https://lore.kernel.org/r/20230118074227.1665098-5-carlos.song@nxp.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>