mac80211: really fix monitor mode channel reporting

After Felix's patch it was still broken in case you
used more than just a single monitor interface. Fix
it better now.

Reported-by: Sujith Manoharan <sujith@msujith.org>
Tested-by: Sujith Manoharan <sujith@msujith.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>