USB: serial: iuu_phoenix: fix memory corruption

commit e7b931be upstream.

The driver would happily overwrite its write buffer with user data in
256 byte increments due to a removed buffer-space sanity check.

Fixes: 5fcf62b0

 ("tty: iuu_phoenix: fix locking.")
Cc: stable <stable@vger.kernel.org>     # 2.6.31
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>