iface-modem-3gpp: fix warnings with -Wswitch-default

mm-iface-modem-3gpp.c: In function ‘handle_register_auth_ready’:
  mm-iface-modem-3gpp.c:550:5: error: switch missing default case [-Werror=switch-default]
    550 |     switch (modem_state) {
        |     ^~~~~~
  mm-iface-modem-3gpp.c: In function ‘handle_scan_auth_ready’:
  mm-iface-modem-3gpp.c:738:5: error: switch missing default case [-Werror=switch-default]
    738 |     switch (modem_state) {
        |     ^~~~~~
  mm-iface-modem-3gpp.c: In function ‘interface_disabling_step’:
  mm-iface-modem-3gpp.c:1788:5: error: switch missing default case [-Werror=switch-default]
   1788 |     switch (ctx->step) {
        |     ^~~~~~
  mm-iface-modem-3gpp.c: In function ‘interface_enabling_step’:
  mm-iface-modem-3gpp.c:2092:5: error: switch missing default case [-Werror=switch-default]
   2092 |     switch (ctx->step) {
        |     ^~~~~~
  mm-iface-modem-3gpp.c: In function ‘interface_initialization_step’:
  mm-iface-modem-3gpp.c:2394:5: error: switch missing default case [-Werror=switch-default]
   2394 |     switch (ctx->step) {
        |     ^~~~~~

src/mm-iface-modem-3gpp.c

@@ -631,6 +631,9 @@ handle_register_auth_ready (MMBaseModem *self,
                                                "Cannot register modem: "
                                                "modem is connected");
         break;
+
+    default:
+        g_assert_not_reached ();
     }
 
     handle_register_context_free (ctx);
@@ -803,6 +806,9 @@ handle_scan_auth_ready (MMBaseModem *self,
             (GAsyncReadyCallback)handle_scan_ready,
             ctx);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 
     handle_scan_context_free (ctx);
@@ -1888,6 +1894,9 @@ interface_disabling_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 
     g_assert_not_reached ();
@@ -2194,6 +2203,9 @@ interface_enabling_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();
@@ -2490,6 +2502,9 @@ interface_initialization_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();