pinctrl: rockchip: correctly handle arguments of pinconf options

Change the rockchip pinctrl driver to handle the arguments to the pull
pinconfig options correctly. So only accept non-0 values for the
pull options as the rockchip pin-controller can only turn pulls on and
off (this via BIAS_DISABLE).

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>