leds-gpio: fix possible crash on OF device unbinding

If there are leds present in the OF tree, but the GPIOs for (some) of
them are unavailable, led_data doesn't get populated with correct
devices. Then, on device unbinding, one can crash the kernel.

Workaround this by setting led->gpio to invalid value early.

Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>