Fix 5 bugs:

1. always call fxp_stop in fxp_detach. Since we don't read from

	   the card, there's no need to carefully look at things with
	   bus_child_present.

2. Call FXP_UNLOCK() before calling bus_teardown_intr to avoid

	   a possible deadlock reported by jhb.

3. add gone to the softc. Set it to true in detach.
4. Return immediately if gone is true in fxp_ioctl
5. Return immediately if gone is true in fxp_intr