staging: dgnc: fixes tty_port and tty_struct params

The functions tty_flip_buffer_push, tty_insert_flip_string,
tty_insert_flip_char and tty_buffer_request_room now require a struct of
type tty_port instead of struct tty_struct. This patch makes those
changes.

Signed-off-by: Lidza Louina <lidza.louina@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>