mac80211: update new sta's rx timestamp

This patch fixes needless probe request caused by zero value in
sta->last_rx inside ieee80211_associated flow

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>