crypto: drbg - use full CTR AES for update

The CTR DRBG update function performs a full CTR AES operation including
the XOR with "plaintext" data. Hence, remove the XOR from the code and
use the CTR mode to do the XOR.

Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>