serial: allow plugins to specify probe-time send_delay

Let modems we know don't suck use a zero send-delay at probe time,
which greatly reduces time required to probe AT-compatible ports.

plugins/mm-plugin-simtech.c

@@ -97,7 +97,7 @@ supports_port (MMPluginBase *base,
     }
 
     /* Otherwise kick off a probe */
-    if (mm_plugin_base_probe_port (base, task, NULL))
+    if (mm_plugin_base_probe_port (base, task, 100000, NULL))
         return MM_PLUGIN_SUPPORTS_PORT_IN_PROGRESS;
 
     return MM_PLUGIN_SUPPORTS_PORT_UNSUPPORTED;