bhyve: tpm: Avoid printing a message when clearing the cancel bit

Some drivers do this routinely, e.g., FreeBSD's tpm20 does this every
time it sends a command in tpmcrb_transmit(). This causes the console
to fill up with messages. Instead, only print a warning if the cancel
bit is set to one.

Reviewed by: corvink
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D52425