staging: dgnc: rename dgnc_tty_uninit() to dgnc_cleanup_tty()

The dgnc_tty_uninit() doesn't match with dgnc_tty_init() at all.
And also the dgnc_cleanup_tty() is only called for exiting the module.

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