mac80211: Fix ibss station got expired immediately

Station addition in ieee80211_ibss_rx_queued_mgmt is not updating
sta->last_rx which is causing station expiry in ieee80211_ibss_work
path. So sta addition and deletion happens repeatedly.

CC: stable@kernel.org
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>