staging:iio:adis16260: Use adis library

Use the new adis library for the adis16260 driver. This allows us to completely
scrap the adis16260 buffer and trigger code and about half of the core driver
code.

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