gsupplicant: Set the scan callback in g_supplicant_interface_scan

gsupplicant: Set the scan callback in g_supplicant_interface_scan

The callback is required to properly handle scanning errors reported
by wpa_supplicant through the scan_done signal.

Steps to reproduce:
1. Set a country code to the WiFi card which have more frequencies
   allowed than the world roaming allow.
2. Connect to a WiFi network not available in the world roaming setup.
3. Stop connman.
4. Change the country code to 00, be sure the frequencies use in step 2.
   is no longer allowed. Take care of cfg80211 authorize frequencies when
   beacons are received, I prefer using mac80211_hwsim from here to avoid
   side effects about beacon reception.
5. Start connman again.

Now ConnMan is stuck in scanning state as the scan_fast method of
plugins/wifi.c is called with a forbidden frequency.

wpa_supplicant returns an error received in "gsupplicant/supplicant.c:
signal_scan_done", where the code enters 'if (success == FALSE)' but
unfortunately scan_callback is NULL at this point as the scan_callback is
normally set in interface_scan_result().