crypto: echainiv - Use aead_register_instance

New style AEAD instances must use aead_register_instance.  This
worked by chance because aead_geniv_alloc is still setting things
the old way.

This patch converts the template over to the create model where
we are responsible for instance registration so that we can call
the correct function.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>