gpio: sysfs: fix memory leak in gpiod_export_link

Fix memory leak in the gpio sysfs interface due to failure to drop
reference to device returned by class_find_device when creating a link.

Fixes: a4177ee7 ("gpiolib: allow exported GPIO nodes to be named
using sysfs links")
Cc: stable <stable@vger.kernel.org>	# v2.6.32
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>