n_hdlc: remove cached tty

It's not needed, as now it's clear, that it's always the same as the one
passed from the tty layer.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084118.26491-13-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>