tty: n_tty: use do-while in n_tty_check_{,un}throttle()

This change gets rid of the complicated exit from the loops. It can be
done much easier using do-while loops.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230919085156.1578-6-jirislaby@kernel.org


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