iface-modem: fix warnings with -Wswitch-default

mm-iface-modem.c: In function ‘bearer_status_changed’:
  mm-iface-modem.c:483:9: error: switch missing default case [-Werror=switch-default]
    483 |         switch (mm_base_bearer_get_status (bearer)) {
        |         ^~~~~~
  mm-iface-modem.c: In function ‘peridic_signal_check_step’:
  mm-iface-modem.c:1293:5: error: switch missing default case [-Werror=switch-default]
   1293 |     switch (ctx->running_step) {
        |     ^~~~~~
  mm-iface-modem.c: In function ‘interface_enabling_step’:
  mm-iface-modem.c:3965:5: error: switch missing default case [-Werror=switch-default]
   3965 |     switch (ctx->step) {
        |     ^~~~~~
  mm-iface-modem.c: In function ‘interface_initialization_step’:
  mm-iface-modem.c:4708:5: error: switch missing default case [-Werror=switch-default]
   4708 |     switch (ctx->step) {
        |     ^~~~~~

src/mm-iface-modem.c

@@ -493,6 +493,8 @@ bearer_status_changed (MMBaseBearer *bearer,
         case MM_BEARER_STATUS_DISCONNECTED:
             new_state = get_consolidated_subsystem_state (self);
             break;
+        default:
+            g_assert_not_reached ();
         }
 
         mm_iface_modem_update_state (self,
@@ -1362,6 +1364,9 @@ peridic_signal_check_step (MMIfaceModem *self)
                                                      (GSourceFunc) periodic_signal_check_cb,
                                                      self);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 }
 
@@ -4051,6 +4056,9 @@ interface_enabling_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();
@@ -5283,6 +5291,9 @@ interface_initialization_step (GTask *task)
 
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();