simtech: fix warnings with -Wswitch-default

simtech/mm-broadband-modem-simtech.c: In function ‘enable_unsolicited_events_context_step’:
  simtech/mm-broadband-modem-simtech.c:378:5: error: switch missing default case [-Werror=switch-default]
    378 |     switch (ctx->step) {
        |     ^~~~~~
  simtech/mm-broadband-modem-simtech.c: In function ‘disable_unsolicited_events_context_step’:
  simtech/mm-broadband-modem-simtech.c:563:5: error: switch missing default case [-Werror=switch-default]
    563 |     switch (ctx->step) {
        |     ^~~~~~

plugins/simtech/mm-broadband-modem-simtech.c

@@ -445,6 +445,9 @@ enable_unsolicited_events_context_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 }
 
@@ -602,6 +605,9 @@ disable_unsolicited_events_context_step (GTask *task)
         g_task_return_boolean (task, TRUE);
         g_object_unref (task);
         return;
+
+    default:
+        g_assert_not_reached ();
     }
 }