pinctrl/lantiq: fix pin number in ltq_pmx_gpio_request_enable

The mapping logic inside ltq_pmx_gpio_request_enable() was
broken. This only effected Falcon SoC.

Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>
Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>