umass: fix a cam_sim leak in error case

While debugging a hang I noticed that in case of error in
umass_cam_attach_sim() we miss a cam_sim_free() call.
Added that to not leak resources.

Reviewed by:	hselasky
MFC after:	3 days
Differential Revision:	D26287

sys/dev/usb/storage/umass.c

@@ -2094,6 +2094,7 @@ umass_cam_attach_sim(struct umass_softc *sc)
 
 	if (xpt_bus_register(sc->sc_sim, sc->sc_dev,
 	    sc->sc_unit) != CAM_SUCCESS) {
+		cam_sim_free(sc->sc_sim, /* free_devq */ TRUE);
 		mtx_unlock(&sc->sc_mtx);
 		return (ENOMEM);
 	}