iio: imu: st_lsm6dsx: make IIO_CHAN_INFO_SCALE shared by type

in_accel_x_scale, in_accel_y_scale and in_accel_z_scale are always
the same. The scale is still defined to be in "info_mask_separate".

Userspace (iio-sensor-proxy and others) is not used to that and only
looks for "in_accel_scale" for the scaling factor to apply.

Change IIO_CHAN_INFO_SCALE from being separate in all channel to be
shared by type.

This removes in_accel_x_scale, in_accel_y_scale and in_accel_z_scale and
makes available in_accel_scale.

This is an ABI change, but hopefully in the 'no one will notice'
category.  The cleanup in interface is worth the small risk that
there is a custom script out there somewhere that this will break.

Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>