pinctrl: at91: initialize config parameter to 0

When passing a not initialized config parameter, at91_pinconf_get() would return
a bogus value. Fix that by initializing it to zero before using it.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>