ipmi: Fixes for ipmi_opal(powernv)
- Crank the OPAL state machine during the receive loop, to make sure the pollers are executed
- Add a proper detach function, so the module can be unloaded and reloaded at runtime.
It still doesn't reliably work 100% of the time on POWER9, and it appears
timing and/or cache related. It may work on POWER8 now.
MFC after: 2 weeks