mac80211: change beacon/connection polling

Since when we detect beacon lost we do active AP probing (using nullfunc
frame or probe request) there is no need to have beacon polling. Flags
IEEE80211_STA_BEACON_POLL seems to be used just for historical reasons.

Change also make that after we start connection poll due to beacon loss,
next received beacon will abort the poll.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>