pinctrl: mediatek: adjust error code and message when some register not supported is found

It's usual and not an error for there's some register not supported by a
certain SoC or a pin so that in the case we have to adjust the message to
print and the error code to get rid of unnecessary false alarm.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>