pinctrl: at91: fix get_pullup/down function return

In PIO_PUSR and PIO_PPDSR register if a given bit is set 1 this means the
pullup/down for this pin (pin is represented as a bit position) is
disabled.

Cc: stable <stable@vger.kernel.org> # 3.8+
Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>