[PowerPC64] Fix OPAL IPMI driver

This change fixes a couple of issues with OPAL IPMI driver and
implements a mechanism to detect timeouts and discard old messages left
in receive queue, to avoid old messages from being confused with the
reply of new ones.

Reviewed by: jhibbits
Sponsored by: Eldorado Research Institute (eldorado.org.br)
Differential Revision: https://reviews.freebsd.org/D24185