staging: vme: use gpiochip data pointer

This makes the driver use the data pointer added to the gpio_chip
to store a pointer to the state container instead of relying on
container_of().

Cc: Manohar Vanga <manohar.vanga@gmail.com>
Cc: devel@driverdev.osuosl.org
Acked-by: Martyn Welch <martyn@welchs.me.uk>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>