crypto: aesni - Use crypto_cipher to derive rfc4106 subkey

Currently aesni uses an async ctr(aes) to derive the rfc4106
subkey, which was presumably copied over from the generic rfc4106
code.  Over there it's done that way because we already have a
ctr(aes) spawn.  But it is simply overkill for aesni since we
have to go get a ctr(aes) from scratch anyway.

This patch simplifies the subkey derivation by using a straight
aes cipher instead.

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