Set sc_dying to 1 when detaching. In NetBSD and OpenBSD this was

done by the DVACT_DEACTIVATE case in *hci_activate(), but we don't
use that code in FreeBSD so it was never set.

sys/dev/usb/ehci.c

@@ -935,6 +935,8 @@ ehci_detach(struct ehci_softc *sc, int flags)
 
 	if (rv != 0)
 		return (rv);
+#else
+	sc->sc_dying = 1;
 #endif
 
 	EOWRITE4(sc, EHCI_USBINTR, sc->sc_eintrs);

sys/dev/usb/ohci.c

@@ -393,6 +393,8 @@ ohci_detach(struct ohci_softc *sc, int flags)
 
 	if (rv != 0)
 		return (rv);
+#else
+	sc->sc_dying = 1;
 #endif
 
 	usb_uncallout(sc->sc_tmo_rhsc, ohci_rhsc_enable, sc);

sys/dev/usb/uhci.c

@@ -585,6 +585,8 @@ uhci_detach(struct uhci_softc *sc, int flags)
 
 	if (rv != 0)
 		return (rv);
+#else
+	sc->sc_dying = 1;
 #endif
 
 	UWRITE2(sc, UHCI_INTR, 0);		/* disable interrupts */