crypto: drbg - fix sparse warning for cpu_to_be[32|64]

The sparse tool complained that the cpu_to_be[32|64] functions return
__be[32|64] instead of __u32 or __u64. The patch replaces the __u32 and
__u64 with __be32 and __be64.

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>