core: use a default timeout of 300s for the scan networks operation

This is the value which we actually suggest in the manpage for the mmcli
operation, so just use the same one.

   Scanning for 3GPP networks may really take a long time, so a specific timeout must be given:
       $ mmcli -m 0 --3gpp-scan --timeout=300

       Found 4 networks:
       21404 - Yoigo (umts, available)
       21407 - Movistar (umts, current)
       21401 - vodafone ES (umts, forbidden)
       21403 - Orange (umts, forbidden)

https://bugs.freedesktop.org/show_bug.cgi?id=98235

plugins/novatel/mm-broadband-modem-novatel-lte.c

@@ -631,7 +631,7 @@ scan_networks (MMIfaceModem3gpp *self,
 
     mm_base_modem_at_command (MM_BASE_MODEM (self),
                               "+COPS=?",
-                              120,
+                              300,
                               FALSE,
                               (GAsyncReadyCallback)cops_query_ready,
                               result);

src/mm-broadband-modem-mbim.c

@@ -2876,7 +2876,7 @@ modem_3gpp_scan_networks (MMIfaceModem3gpp *self,
     message = mbim_message_visible_providers_query_new (MBIM_VISIBLE_PROVIDERS_ACTION_FULL_SCAN, NULL);
     mbim_device_command (device,
                          message,
-                         120,
+                         300,
                          NULL,
                          (GAsyncReadyCallback)visible_providers_query_ready,
                          result);

src/mm-broadband-modem-qmi.c

@@ -4325,7 +4325,7 @@ modem_3gpp_scan_networks (MMIfaceModem3gpp *self,
     mm_dbg ("Scanning networks...");
     qmi_client_nas_network_scan (QMI_CLIENT_NAS (client),
                                  NULL,
-                                 100,
+                                 300,
                                  NULL,
                                  (GAsyncReadyCallback)nas_network_scan_ready,
                                  result);

src/mm-broadband-modem.c

@@ -3753,7 +3753,7 @@ modem_3gpp_scan_networks (MMIfaceModem3gpp *self,
 {
     mm_base_modem_at_command (MM_BASE_MODEM (self),
                               "+COPS=?",
-                              120,
+                              300,
                               FALSE,
                               callback,
                               user_data);