staging: dgap: tty.c: fixes errors with tty function calls

This patch fixes errors with the tty function calls
tty_buffer_request_room, tty_insert_flip_string_flags,
tty_insert_flip_string and tty_flip_buffer_push.

They now take struct tty_port as a parameter instead
of tty_struct.

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