usbnet(9): Make sure unp->unp_if_flags is initialized on init.

usbnet_ifflags_cb is only called if the flags change while up and
running.  (XXX Maybe it should be called in other circumstances too
so there's only one path here?)

Out of paranoia, clear the cache on stop.

PR kern/57645

XXX pullup-10

(riastradh)