tty: n_tty: use 'num' for writes' counts

We have a separate misnomer 'c' to hold the retuned value from
tty->ops->write(). Instead, use 'num' already defined on another place
(and already properly typed).

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230827074147.2287-4-jirislaby@kernel.org


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