pinctrl: rockchip: add return value to rockchip_set_mux

In a following change, rockchip_set_mux gets the possibility to fail.
Therefore add a return value to it and honor error codes in functions
using rockchip_set_mux.

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