tty: use min() in iterate_tty_write()

It simplifies the code. The "price" is we have to unify 'chunk' to be
size_t the same as 'count' is. But that change is actually correct.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-24-jirislaby@kernel.org


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