n_hdlc: expand tty2n_hdlc macro

It's simple tty->disc_data, but it obfuscates code. So expand it to all
locations and drop it.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084118.26491-11-jslaby@suse.cz


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