crypto: aesni - make AVX2 AES-GCM work with any aadlen

This is the first step to make the aesni AES-GCM implementation
generic. The current code was written for rfc4106, so it handles only
some specific sizes of associated data.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>