bhyve: do not hold CRB mutex when executing TPM commands

TPM commands can take up to several seconds to execute. If we hold the
CRB mutex while executing the command, MMIO accesses could be blocked
for a long time. Therefore, just copy all required values and work on
the copied values.

Reviewed by: markj
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D40724