broadband-modem-qmi: fix reporting of signal strength indications

We had all the logic in place... but we were never actually enabling
the signal strength indications because the `enable` flag in the
context was never set.

The bug was introduced back in May 2018, and released with 1.10.0.

Fixes baefe53ab9

src/mm-broadband-modem-qmi.c

@@ -4909,6 +4909,7 @@ common_enable_disable_unsolicited_events (MMBroadbandModemQmi *self,
     }
 
     ctx = g_new0 (EnableUnsolicitedEventsContext, 1);
+    ctx->enable = enable;
     ctx->client = g_object_ref (client);
 
     g_task_set_task_data (task, ctx, (GDestroyNotify)enable_unsolicited_events_context_free);