bhyve: don't crash when guest writes TPM int_enable register

FreeBSD's own TPM driver writes 0 to the INT_ENABLE register during
attach, making sure interrupts are off. bhyve really shouldn't kill the
VM when that happens just because it doesn't support TPM interrupts.
Even if a guest was trying to enable interrupts, we should just ignore
it like real TPM hardware would.

Reviewed by: corvink
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D46562