IB/cxgb3: Fix error codes in iwch_alloc_mr()

We accidentally don't set the error code on some error paths.  It means
return ERR_PTR(0) which is NULL and results in a NULL dereference in the
caller.

Fixes: 13a23933 ("RDMA/cxgb3: Don't ignore insert_handle() failures")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>