bearer-mbim: fix warnings with -Wswitch-default

mm-bearer-mbim.c: In function ‘connect_context_step’:
  mm-bearer-mbim.c:840:5: error: switch missing default case [-Werror=switch-default]
    840 |     switch (ctx->step) {
        |     ^~~~~~
  mm-bearer-mbim.c: In function ‘disconnect_context_step’:
  mm-bearer-mbim.c:1266:5: error: switch missing default case [-Werror=switch-default]
   1266 |     switch (ctx->step) {
        |     ^~~~~~
  ...

src/mm-bearer-mbim.c

@@ -1060,6 +1060,9 @@ connect_context_step (GTask *task)
             (GDestroyNotify)mm_bearer_connect_result_unref);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();
@@ -1306,6 +1309,9 @@ disconnect_context_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 }