pinctrl: pinconf: fix comparison of different types

Fix the following sparse warning:

drivers/pinctrl/pinconf.c:521:20: error: incompatible types in comparison expression (different type sizes)

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>