gpio: axp209: use correct register for GPIO input status

The GPIO input status was read from control register
(AXP20X_GPIO[210]_CTRL) instead of status register (AXP20X_GPIO20_SS).

Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>