staging:iio:adis16220: Use adis library

Use the new adis library for the adis16220 driver. The adis16220 driver is a bit
special and so we can only make use of the generic register access and control
functions for now.

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