mac80211: set carrier on for monitor interfaces on ieee80211_open

If a station interface is reused as monitor interface it is possible that
the carrier is still set to off. This breaks packet injection on that
monitor interface.
Force the carrier on in monitor interface initialisation like it is also done
for other interface types (e.g. adhoc, mesh point, ap).

Signed-off-by: David Gnedt <david.gnedt@davizone.at>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>