Restore wpa_s->scan_req if scan trigger fails

Restore wpa_s->scan_req if scan trigger fails

If the driver rejects the scan request while handling an external
requested scan (e.g., SCAN command on ctrl_iface), wpa_s->scan_req gets
cleared in wpa_supplicant_scan(). This can results in issues when the
scheduled re-try of the scan in one seconds trigger another call to this
function. If ap_scan==2 mode is used, this would result in new
association attempt instead of a new scan. Avoid this by restoring value
of wpa_s->scan_req in case the scan trigger fails and a new scan attempt
is scheduled.

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