staging: dgnc: Remove some redundant functions

dgnc_tty_preinit() and dgnc_tty_post_uninit() functions are used to
allocate and free "dgnc_TmpWriteBuf" and this "dgnc_TmpWriteBuf" is
not really getting used. Hence remove these redundant functions.

Also remove dgnc_TmpWriteBuf variable as it is not used anymore.

Signed-off-by: Pankaj Bharadiya <pankaj.bharadiya@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>