Staging: iio: test for failed allocation

We should return test to see if iio_allocate_trigger() fails and return -ENOMEM.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>