staging:iio:adis16080: Move out of staging

The driver is rather simple and in a good shape. It follows the IIO ABI and the
standard codechecker tools do not report any issues, so move it out of staging.

While moving it also remove one outdated 'fixme' comment.

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