staging: iio: adis16240: Use Managed device functions

Use the adis managed device functions to setup the buffer and the trigger.
The ultimate goal will be to completely drop the non devm version from
the lib.

Since we are here, drop the `.remove` callback by further using devm
functions.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20200915120258.161587-10-nuno.sa@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>