staging:iio:adis16209: Use adis library

Use the new adis library for the adis16209 driver. This allows us to completely
scrap the adis16209 buffer and trigger code and more than half of the core
driver code.

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