cinterion: fix warnings with -Wswitch-default

cinterion/mm-broadband-bearer-cinterion.c: In function ‘dial_3gpp_context_step’:
  cinterion/mm-broadband-bearer-cinterion.c:370:5: error: switch missing default case [-Werror=switch-default]
    370 |     switch (ctx->step) {
        |     ^~~~~~
  cinterion/mm-broadband-bearer-cinterion.c: In function ‘disconnect_3gpp_context_step’:
  cinterion/mm-broadband-bearer-cinterion.c:610:5: error: switch missing default case [-Werror=switch-default]
    610 |     switch (ctx->step) {
        |     ^~~~~~

plugins/cinterion/mm-broadband-bearer-cinterion.c

@@ -454,6 +454,9 @@ dial_3gpp_context_step (GTask *task)
         g_task_return_pointer (task, g_object_ref (ctx->data), g_object_unref);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 }
 
@@ -648,6 +651,9 @@ disconnect_3gpp_context_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 }