let ieee80211_recv_mgmt() return 0 if the frame is to be discarded and don't

let ieee80211_recv_mgmt() return 0 if the frame is to be discarded and don't
process discarded frames in ath_recv_mgmt(). this reduces the number of times
we attempt to merge IBSS to invalid beacons.

this patch is based on the work of Derek Smithies

git-svn-id: http://madwifi-project.org/svn/madwifi/trunk@3634 0192ed92-7a03-0410-a25b-9323aeb14dbd