gpio: Avoid using an invalid ofnode

Devices do not necessarily have nodes attached to them, since they can be
created from platdata. In SPL a devicetree may in fact not exist at all.

Check the node before using it. This avoids failure when OF_CHECKS is
enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 5fc7cf8c ("gpio: add gpio-hog support")
Reviewed-by: Heiko Schocher <hs@denx.de>