gpio: sysfs: fix memory leak in gpiod_sysfs_set_active_low

Fix memory leak in the gpio sysfs interface due to failure to drop
reference to device returned by class_find_device when setting the
gpio-line polarity.

Fixes: 07697461 ("gpiolib: add support for changing value polarity
in sysfs")
Cc: stable <stable@vger.kernel.org>	# v2.6.33
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>