staging:iio:meter:ade7758: Update trigger to the new API

Update trigger to the new API.
Add file comment/license header.

Some backporting needed to keep the driver building all the way
through the core changes.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>