gpio: Fix debug message in of_get_named_gpio_flags()

This was probably missed in the conversion done in commit 3d0f7cf0
("gpio: Adjust of_xlate API to support multiple GPIO chips").

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>