iio: trigger: Add filter callback

Allow devices to reject triggers and vice versa.

Changes since V1:
Added kernel-doc
Moved callback into iio_info
Changed function naming
Revised return value passing
Add forward declaration to avoid warnings

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>