staging: dgap: rename dgap_tty_uninit() to dgap_cleanup_tty()

- dgap_tty_uninit() doesn't match dgap_tty_init() at all.
so rename it. It is just used for cleanup when this module is
exited or failed to initialize by dgap_init_module.

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