HID: nvidia-shield: Fix a missing led_classdev_unregister() in the probe error handling path

The commit in Fixes updated the error handling path of
thunderstrike_create() and the remove function but not the error handling
path of shield_probe(), should an error occur after a successful
thunderstrike_create() call.

Add the missing call. Make sure it is safe to call in the probe error
handling path by preventing the led_classdev from attempting to set the LED
brightness to the off state on unregister.

Fixes: f88af60e

 ("HID: nvidia-shield: Support LED functionality for Thunderstrike")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>