ctl: Always return errno values from ctl_port_register

Return EBUSY instead of a bare 1 if a port number is already active.

Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D42929

sys/cam/ctl/ctl_frontend.c

@@ -162,7 +162,7 @@ ctl_port_register(struct ctl_port *port)
 	if ((port_num < 0) ||
 	    (ctl_set_mask(softc->ctl_port_mask, port_num) < 0)) {
 		mtx_unlock(&softc->ctl_lock);
-		return (1);
+		return (EBUSY);
 	}
 	softc->num_ports++;
 	mtx_unlock(&softc->ctl_lock);