gpio: ks8695: remove irq_to_gpio function

The ks8695 gpio driver has its own copy of the irq_to_gpio()
function. This is completely unused in the mainline kernel
after we converted all remaining users several years ago,
so we can remove the definition as well.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>