crypto: drbg - reduce number of setkey calls

The CTR DRBG code always set the key for each sym cipher invocation even
though the key has not been changed.

The patch ensures that the setkey is only invoked when a new key is
generated by the DRBG.

With this patch, the CTR DRBG performance increases by more than 150%.

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