Fix for powering off a HP DJ950C during printing. As stated by Ian:

When the printer is turned off the pipe write will cause and error,
which causes lpd to close the device and reopen it to clear the error.
After a short while the device will disappear from the bus but lpd will
have opened the ulpt0 port by then. ulpt_status will check for status
without checking the sc->dying flag and panic the kernel when the device
finally disappears from the bus.

Submitted by:	Ian Dowse <iedowse@maths.tcd.ie>

sys/dev/usb/ulpt.c

@@ -427,6 +427,11 @@ ulptopen(dev, flag, mode, p)
 			sc->sc_state = 0;
 			return (error);
 		}
+
+		if (sc->sc_dying) {
+			sc->sc_state = 0;
+			return (ENXIO);
+		}
 	}
 
 	err = usbd_open_pipe(sc->sc_iface, sc->sc_bulk, 0, &sc->sc_bulkpipe);