pinctrl: nomadik: fix up typo

Commit a60b57eddaa8af6c02cf7bbeb58ebf82881f08ac
"drivers/gpio: gpio-nomadik: Add support for irqdomains"
changed GPIO offset calculations to have this form:
(gpio % NMK_GPIO_PER_CHIP) except in this one place for
setting sleep mode, where the conversion was all wrong, and
instead mod:ing the GPIO with the IRQ base which does not
make any sense.

So fix this up so we can use sleepmode.

Reviewed-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>