tty: n_tty: make n_tty_data::num_overrun unsigned

n_tty_data::num_overrun is unlikely to overflow in a second. But make it
explicitly unsigned to avoid printing negative values.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230827074147.2287-6-jirislaby@kernel.org


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