tpm20: Preserve all harvested random bytes

[kbowling]

Summary

The TPM requests 16 random bytes every ten seconds, but a random
harvest event retains at most eight bytes.  random_harvest_queue()
reduces a larger input to one 32-bit Jenkins hash, so each successful
TPM request contributed only four bytes to the entropy accumulator.

Split the response into harvest-event-sized chunks.  This preserves all
16 random bytes without increasing the TPM command rate or changing its
high-quality source classification.

MFC after:      2 weeks