Fix monitor mode on iwm(4) and iwx(4).

Fix monitor mode on iwm(4) and iwx(4).

Set sc->ns_nstate when bypassing the driver's newstate handler while
bringing the interface down.
Otherwise, if the interface was already running, the INIT->RUN transition
which kicks off monitor mode will appear as a RUN->RUN transition to the
driver's newstate handler, and the handler will do nothing.
Monitor mode did not work as a result.