core: when probing mark ports as AT capable too

On a ZTE MF626, sometimes the aux port will respond only with
"ERROR" to probing commands (while the SIM is starting up) and
previously we'd lose the port because we were only looking for
valid probe responses.  But if the port returns ERROR or CME ERROR
etc we know it's an AT port and that we can use it once we've
gotten the type response (CDMA or GSM) from the main port.

src/mm-plugin-base.h

@@ -38,6 +38,7 @@
 #define MM_PLUGIN_BASE_PORT_CAP_IS856       0x0100 /* CDMA 3G EVDO rev 0 */
 #define MM_PLUGIN_BASE_PORT_CAP_IS856_A     0x0200 /* CDMA 3G EVDO rev A */
 #define MM_PLUGIN_BASE_PORT_CAP_QCDM        0x0400 /* QCDM-capable port */
+#define MM_PLUGIN_BASE_PORT_CAP_AT          0x0800 /* Responds to AT commands */
 
 #define MM_TYPE_PLUGIN_BASE_SUPPORTS_TASK            (mm_plugin_base_supports_task_get_type ())
 #define MM_PLUGIN_BASE_SUPPORTS_TASK(obj)            (G_TYPE_CHECK_INSTANCE_CAST ((obj), MM_TYPE_PLUGIN_BASE_SUPPORTS_TASK, MMPluginBaseSupportsTask))