crypto: drbg - leave cipher handles operational

As the DRBG does not operate on shadow copies of the DRBG instance
any more, the cipher handles only need to be allocated once during
initalization time and deallocated during uninstantiate time.

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