crypto: jitter - Add clarifying comments to Jitter Entropy RCT cutoff values

The RCT cutoff values are correct, but they don't exactly match the ones
one would expect when computing them using the formula in SP800-90B. This
discrepancy is due to the fact that the Jitter Entropy RCT starts at 1. To
avoid any confusion by future reviewers, add some comments and explicitly
subtract 1 from the "correct" cutoff values in the definitions.

Signed-off-by: Joachim Vandersmissen <git@jvdsn.com>
Reviewed-by: Stephan Mueller <smueller@chronox.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>