staging:iio: Move adis library out of staging

Now that the adis library no longer depends on the sw_ring buffer implementation
we can move it out of staging.

While we are at it also sort the entries in the iio Kconfig and Makefile to be
in alphabetical order.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>