hwrng: core - allow perfect entropy from hardware devices

Hardware random number quality is measured from 0 (no entropy) to 1024
(perfect entropy). Allow hardware devices to assert the full range by
truncating the device-provided value at 1024 instead of 1023.

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>