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.

src/mm-plugin-base.c

@@ -845,6 +845,7 @@ try_open (gpointer user_data)
 gboolean
 mm_plugin_base_probe_port (MMPluginBase *self,
                            MMPluginBaseSupportsTask *task,
+                           guint64 send_delay_us,
                            GError **error)
 {
     MMPluginBaseSupportsTaskPrivate *task_priv = MM_PLUGIN_BASE_SUPPORTS_TASK_GET_PRIVATE (task);
@@ -869,7 +870,7 @@ mm_plugin_base_probe_port (MMPluginBase *self,
     }
 
     g_object_set (serial,
-                  MM_SERIAL_PORT_SEND_DELAY, (guint64) 100000,
+                  MM_SERIAL_PORT_SEND_DELAY, send_delay_us,
                   MM_PORT_CARRIER_DETECT, FALSE,
                   NULL);
 

src/mm-plugin-base.h

@@ -138,6 +138,7 @@ gboolean mm_plugin_base_get_device_ids (MMPluginBase *self,
 
 gboolean mm_plugin_base_probe_port (MMPluginBase *self,
                                     MMPluginBaseSupportsTask *task,
+                                    guint64 send_delay_us,
                                     GError **error);
 
 /* Returns TRUE if the port was previously probed, FALSE if not */