random(4): Don't complain noisily when an entropy source is slow

Mjg@ reports that RDSEED (r347239) causes a lot of logspam from this printf,
and I don't feel that it is especially useful (even ratelimited). There are
many other quality/quantity checks we're not performing on entropy sources;
lack of high frequency availability does not disqualify a good entropy
source.

There is some discussion in the linked Differential about what logging might
be appropriate and/or polling policy for slower TRNG sources. Please feel
free to chime in if you have opinions.

(cherry picked from commit e01ada5c44c66db8e1f7ed2c1f5622a794a1c43b)