tee: amdtee: skip tee_device_unregister if tee_device_alloc fails

Currently, if tee_device_alloc() fails, then tee_device_unregister()
is a no-op. Therefore, skip the function call to tee_device_unregister() by
introducing a new goto label 'err_free_pool'.

Reported-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>