mac80211: Fix ieee80211_sta_conn_mon_timer with hw connection monitoring

When IEEE80211_HW_CONNECTION_MONITOR is configured by the driver, starting
of ieee80211_sta_conn_mon_timer should be prevented, as it is then not needed.

This is currently partially the case. As it seems, when a probe-response is
received from the AP the timer is still restarted, thus restarting the host
based connection keep-alive mechanism. These probe-responses happen at least
when scanning while associated.

Fix this by preventing starting of the ieee80211_sta_conn_mon_timer in the
ieee80211_rx_mgmt_probe_resp function.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>