serial: allow spew control to be turned off

It's only used during probing where some port types (Sierra CnS
and some Icera devices) may send streams of data that we can't
understand until we close the port.  It interferes with some SMS
operations, so turn it off for ports opened after probing.

src/mm-plugin-base.c

@@ -872,6 +872,7 @@ mm_plugin_base_probe_port (MMPluginBase *self,
     g_object_set (serial,
                   MM_SERIAL_PORT_SEND_DELAY, send_delay_us,
                   MM_PORT_CARRIER_DETECT, FALSE,
+                  MM_SERIAL_PORT_SPEW_CONTROL, TRUE,
                   NULL);
 
     mm_at_serial_port_set_response_parser (serial,