pinctrl: rockchip: remove redundant check

The check limiting bias options to supported ones is already
done thru rockchip_pinconf_pull_valid. Therefore this check is
redundant and can be removed.

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