RDMA/cxgb4: Don't use null ep ptr

In c4iw_modify_qp() error path, only use qhp->ep if ep is not already set.
Otherwise qhp->ep can be NULL and we crash.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>