iio: health/afe440x: Make gain settings a modifier for the stages

Currently the TIA gain settings are exported to userspace as sysfs
entries that do not clearly represent their internal relation to the
sampling stages. The gain settings are enabled on a per-stage basis,
this can be seen in figure 24 of the current AFE4404 datasheet.
These gain settings should therefore be tied to the channels that are
read during these stages. Make this change here.

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>