Clear scan_res_handler on no-retry failure

Clear scan_res_handler on no-retry failure

Previously it was possible for wpa_s->scan_res_handler to remain set to
its old value in case wpa_drv_scan() failed and no retry for the scan
trigger was scheduled (i.e., when last_scan_req == MANUAL_SCAN_REQ).
This could result in getting stuck with the next connection attempt
after a failed "SCAN TYPE=ONLY" operation when wpa_s->scan_res_handler
was set to scan_only_handler().

Fix this by clearing wpa_s->scan_res_handler if wpa_drv_scan() fails and
no retry is scheduled.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>