ipmi: correctly handle ipmb requests
Handle IPMB requests using SEND_MSG (sent as driver request as we do not
need to return anything back to userland for this) and GET_MSG (sent as
usual request so we can return the data for RECEIVE_MSG ioctl) pair.
This fixes fetching complete sensor data from boards (e.g. HP ProLiant
Reviewed by: philip
Differential Revision: https://reviews.freebsd.org/D35605