mac80211: Fix race in checking AP status by sending null frame

mac80211 tries to verify the existence of the current AP by
probing or sending a NULL frame in function
ieee80211_mgd_probe_ap_send. It 1st sends a null frame to the AP,
increments probe_send_count and waits for the ACK to the NULL
frame for a finite duration of time. At times, it happens that by
the time mac80211 gets to increment probe_send_count, the ACK for
the NULL frame transmitted has already been processed. This leads
to a race condition where mac80211 times out waiting for the ACK
for the NULL frame causing unnecessary disconnection with the AP.

Signed-off-by: Soumik Das <soumik.das@stericsson.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>