gpiolib: Use IRQ hardware number getter instead of direct access

IRQ framework provides special type and getter to transform
the Linux IRQ to the hardware pin. Use that type and getter
function instead of direct access.

While at it, amend an indentation in a couple of places.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>