In order to conserve CPU, the read(2) syscall would schedule_timeout
unconditionally.  This also crippled speed, and was a bad design
decision.  This cset merges the updated read(2) logic of the sister
driver amd768_rng from Alan, which schedules only when it needs to.
  
On my test system, by one microbenmark, read(2) output jumped
from 0.08 kbit/s to "what Intel expects" of 20 kbit/s.
  
End users may notice a significant decrease in idle time after
this change (and a correspondingly large increase in /dev/hwrng user
speed), if /dev/hwrng is used to its maximum capacity.