crypto: drbg - simplify ordering of linked list in drbg_ctr_df

As reported by a static code analyzer, the code for the ordering of
the linked list can be simplified.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>