The driver would happily overwrite its write buffer with user data in
256 byte increments due to a removed buffer-space sanity check.
Fixes: 5fcf62b0f1 ("tty: iuu_phoenix: fix locking.")
Cc: stable <stable@vger.kernel.org> # 2.6.31
Signed-off-by: Johan Hovold <johan@kernel.org>