i2c: mux: gpio: don't fiddle with GPIOLIB internals

Use the relevant API functions to retrieve the address of the
underlying struct device instead of accessing GPIOLIB private structures
manually.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Acked-by: Peter Rosin <peda@axentia.se>
Acked-by: Wolfram Sang <wsa@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>