pinctrl: mcp23s08: switch to devm_gpiochip_add_data

Switching to devm_gpiochip_add_data simplifies the driver's
cleanup routine and safes a few loc.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>