iio: health/afe440x: Always use separate gain values

Locking the two gain stages to the same setting adds no value for us,
so initialize them as unlocked and remove the sysfs for unlocking them.
This also allows us to greatly simplify showing and setting the gain
registers.

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