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
DL380 Gen10).

Reviewed by: philip
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D35605