port-serial-at: make mm_port_serial_at_command_finish() transfer ownership

This will make it slightly easier to port mm_port_serial_at_command() to
use GTask, since we won't have to keep a pointer to the result in GTask
after _finish() has been called.

In most cases the caller needs the value anyway, so this doesn't add too
much hasle.

src/plugins/cinterion/mm-plugin-cinterion.c

@@ -80,7 +80,7 @@ sqport_ready (MMPortSerialAt *port,
               GTask          *task)
 {
     MMPortProbe *probe;
-    const gchar *response;
+    g_autofree gchar *response = NULL;
 
     probe = g_task_get_source_object (task);