iio: trigger: clean up viio_trigger_alloc()

1) Reverse the test for kmalloc() failure so we can pull everything
   back one tab.
2) Use gotos for unwinding.
3) Some of the extra line breaks for the 80 character limit are no
   longer needed so we can remove them.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>