tpm20: Support partial reads

Summary:
In some cases the TPM utilities may read only a partial block, instead
of a full block. If a new command starts while in the middle of a read
it may cause the TPM to go catatonic and no longer respond to SPI.

Reviewed by: kd
Obtained from: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D45140