mac80211: fix connection polling

Commit 392b9ffb ("mac80211: change beacon/connection polling")
removed the IEEE80211_STA_BEACON_POLL flag.

However, it accidentally removed the setting of
IEEE80211_STA_CONNECTION_POLL, making the connection polling
completely useless (the flag is always clear, so the result
is never being checked). Fix it.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>