RDMA/cxgb4: Protect from possible dereference

Smatch tool reports the following error:
  drivers/infiniband/hw/cxgb4/qp.c:1886
	c4iw_create_qp() error: we previously assumed 'ucontext'
	could be null (see line 1804)

Cc: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>