pinctrl: at91: disable PD or PU before enabling PU or PD

Disable Pull-Down or Pull-Up property before enabling Pull-Up or
Pull-Down, because the pin's Pull-Up and Pull-Down property is
mutually exclusive.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>