staging:iio: Move ad799x driver out of staging

The driver is now at a reasonable quality level. Move it out of staging.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>