staging: dgap: remove unneeded kfree() in dgap_tty_register_ports()

When it failed to allocate for printer_ports, serial_ports
can be freed in dgap_tty_uninit().

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