tty: Fix direct use of tty buffer work

Recent abstraction of tty buffer work introduced api to manage
tty input kworker; use it.

Fixes: e176058f ("tty: Abstract tty buffer work")
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>