staging:iio:adc:ad7887: Convert to new channel registration method.

Convert to new channel registration method
Update / change license copyright header
Add missing call to iio_trigger_notify_done()

V2: use IIO_CHAN macro.

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