ums(4): Start USB xfers on opening of evdev node unconditionally.

This fixes inability to start USB xfers in a case when FIFO has been
already open()-ed but no read() or poll() calls has been issued yet.

MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D30343

sys/dev/usb/input/ums.c

@@ -952,8 +952,8 @@ ums_ev_open(struct evdev_dev *evdev)
 
 	if (sc->sc_fflags == 0) {
 		ums_reset(sc);
-		ums_start_rx(sc);
 	}
+	ums_start_rx(sc);
 
 	return (0);
 }