sierra: early detect non-AT ports

Detect the parser filter errors during our custom_init(), and set the port as
not being AT if the error found.

plugins/sierra/mm-plugin-sierra.c

@@ -86,6 +86,12 @@ gcap_ready (MMAtSerialPort *port,
             g_error_matches (error, MM_SERIAL_ERROR, MM_SERIAL_ERROR_RESPONSE_TIMEOUT)) {
             mm_port_probe_set_result_at (ctx->probe, FALSE);
         }
+        /* If reported a hard parse error, this port is definitely not an AT
+         * port, skip trying. */
+        else if (g_error_matches (error, MM_SERIAL_ERROR, MM_SERIAL_ERROR_PARSE_FAILED)) {
+            mm_port_probe_set_result_at (ctx->probe, FALSE);
+            ctx->retries = 0;
+        }
 
         /* Just retry... */
         sierra_custom_init_step (ctx);