rtwn: do not start vap when initialization fails

• Start vap(s) (via ieee80211_start_all()) only when initialization

succeeds; stop the first vap otherwise (via ieee80211_stop());

• Do not try to stop a device multiple times

(move (sc->sc_flags & RTWN_RUNNING) check to urtwn_stop_locked()).

Tested by: kevlo
Reviewed by: kevlo
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D5058