cxgbe(4): Allow tracing filters on loopback ports.

Each physical port has an associated loopback tx channel and anything
transmitted over that channel by the driver is looped back internally by
the hardware as if received on that physical port.  This change allows
tracing filters to be installed in this loopback path.

MFC after:	1 week
Sponsored by:	Chelsio Communications

sys/dev/cxgbe/t4_tracer.c

@@ -388,7 +388,8 @@ t4_set_tracer(struct adapter *sc, struct t4_tracer *t)
 			goto done;
 		}
 		tp.port = sc->port[t->tp.port - 4]->tx_chan + 4;
-	}
+	} else
+		tp.port = t->tp.port;
 	tpp = &tp;
 done:
 	if (rc == 0) {