staging: frontier: fix memory leak in usb_alphatrack_probe()

oldi_buffer and write_buffer need to free when usb_alphatrack_delete()
is called.

Signed-off-by: Daeseok Youn <daeseok.youn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>