core,broadband-bearer: fix warnings with -Wswitch-default

mm-broadband-bearer.c: In function ‘cid_selection_3gpp_context_step’:
  mm-broadband-bearer.c:868:5: warning: switch missing default case [-Wswitch-default]
    868 |     switch (ctx->step) {
        |     ^~~~~~
  mm-broadband-bearer.c: In function ‘disconnect’:
  mm-broadband-bearer.c:1842:5: warning: switch missing default case [-Wswitch-default]
   1842 |     switch (MM_BROADBAND_BEARER (self)->priv->connection_type) {
        |     ^~~~~~
  mm-broadband-bearer.c: In function ‘interface_initialization_step’:
  mm-broadband-bearer.c:2124:5: warning: switch missing default case [-Wswitch-default]
   2124 |     switch (ctx->step) {
        |     ^~~~~~

src/mm-broadband-bearer.c

@@ -891,6 +891,9 @@ cid_selection_3gpp_context_step (GTask *task)
         g_task_return_int (task, (gssize) ctx->cid);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 }
 
@@ -1856,6 +1859,7 @@ disconnect (MMBaseBearer *self,
         break;
 
     case CONNECTION_TYPE_NONE:
+    default:
         g_assert_not_reached ();
     }
 
@@ -2144,6 +2148,9 @@ interface_initialization_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 
     g_assert_not_reached ();