Add a cv_wait to the TPM2.0 harvesting function

Harvesting has to compete for the TPM chip with userspace.
Before this change the callout could hijack an unread buffer
causing a userspace call to the TPM to fail.

Submitted by: Kornel Duleba <mindal@semihalf.com>
Reviewed by: delphij
Obtained from: Semihalf
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D19712