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/huawei/mm-plugin-huawei.c

@@ -104,7 +104,7 @@ getportmode_ready (MMPortSerialAt *port,
     MMDevice                *device;
     MMPortProbe             *probe;
     HuaweiCustomInitContext *ctx;
-    const gchar             *response;
+    g_autofree gchar        *response = NULL;
     GArray                  *modes;
     g_autoptr(GError)        error = NULL;