gpio: mcp: 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: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>