as of intel spec rdrand needs >1022 calls for a guaranteed reseed, adjust accordingly

[gebi]

thx for your really nice implementation of rngd-rdrand!

I just fixed a minor thing, that as of intel's spec rdrand only guarantees a reseed after more than 1022 calls to rdrand.

// https://software.intel.com/en-us/articles/intel-digital-random-number-generator-drng-software-implementation-guide
//   "The DRBG autonomously decides when it needs to be reseeded to refresh the
//   random number pool in the buffer and is both unpredictable and transparent
//   to the RDRAND caller. An upper bound of 511 128-bit samples will be
//   generated per seed. That is, no more than 511*2=1022 sequential DRNG
//   random numbers will be generated from the same seed value."