iface-modem-oma: fix warnings with -Wswitch-default

mm-iface-modem-oma.c: In function ‘interface_disabling_step’:
  mm-iface-modem-oma.c:729:5: error: switch missing default case [-Werror=switch-default]
    729 |     switch (ctx->step) {
        |     ^~~~~~
  ...

src/mm-iface-modem-oma.c

@@ -762,6 +762,9 @@ interface_disabling_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();
@@ -960,6 +963,9 @@ interface_enabling_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();
@@ -1142,6 +1148,9 @@ interface_initialization_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        break;
     }
 
     g_assert_not_reached ();