gpio: tps65912: fix wrong container_of arguments

The gpio_chip operations receive a pointer the gpio_chip struct which is
contained in the driver's private struct, yet the container_of call in those
functions point to the mfd struct defined in include/linux/mfd/tps65912.h.

Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Nicolas Saenz Julienne <nicolassaenzj@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>