pinctrl: rockchip: missing unlock on error in rockchip_set_pull()

We need to unlock here before returning -EINVAL.

Fixes: 6ca5274d ('pinctrl: rockchip: add rk3188 specifics')

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>