crypto: mediatek - use AES library for GCM key derivation

The Mediatek accelerator driver calls into a dynamically allocated
skcipher of the ctr(aes) variety to perform GCM key derivation, which
involves AES encryption of a single block consisting of NUL bytes.

There is no point in using the skcipher API for this, so use the AES
library interface instead.

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