mac80211: Reset assoc_scan_tries after an unsuccessful scan run

Trying to associate with a non-existent SSID stops the
state machine after the first run. Subsequent association
requests fail to start the scan engine. Fix this by resetting
assoc_scan_tries to zero after completing a scan run.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>