crypto: omap-aes-gcm - use the AES library to encrypt the tag

The OMAP AES-GCM implementation uses a fallback ecb(aes) skcipher to
produce the keystream to encrypt the output tag. Let's use the new
AES library instead - this is much simpler, and shouldn't affect
performance given that it only involves a single block.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Tero Kristo <t-kristo@ti.com>
Tested-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>