staging:iio:adis16400: Use adis library

Use the new adis library for the adis16400 driver. This allows us to completely
scrap the adis16400 trigger code and more than half of the core driver code. For
now we can not make use of the generic adis buffer implementation since the
adis16400 driver has special requirements due to its burst mode support. But
we will eventually get to this.

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