Add a hack-around to this fatal taskqueue running whilst the NIC
is detaching.

This mostly fixes a panic - the reset path shouldn't run whilst
the NIC is being torn down.

It's not locked, so it's "mostly" ok, but most of the rest of
the driver doesn't read sc->invalid with sensible locking. Grr.

The real solution is to cleanly tear down taskqueues in the detach/suspend
phase, but ..