HID: nvidia-shield: Add missing null pointer checks to LED initialization

devm_kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure. Ensure the allocation was successful
by checking the pointer validity.

[jkosina@suse.com: tweak changelog a bit]
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Reviewed-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>