gpio: handle compatible ioctl() pointers

If we're using the compatible ioctl() we need to handle the
argument pointer in a special way or there will be trouble.

Fixes: 3c702e99 ("gpio: add a userspace chardev ABI for GPIOs")
Reported-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>