crypto: drbg - Fixes panic in wait_for_completion call

Initialise ctr_completion variable before use.

Signed-off-by: Harsh Jain <harshjain.prof@gmail.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>