IB/qib: Cleanup qib_register_observer()

Returning directly is easier to read than do-nothing gotos.  Remove the
duplicative check on "olp" and pull the code in one indent level.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>