tee: amdtee: rename err label to err_device_unregister

Rename err label to err_device_unregister for better
readability.

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Rijo Thomas <Rijo-john.Thomas@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>