ipmi: Fix issues with BMC refcounts

BMC device refcounts were not being decremented after fetching from
driver_find_device().  Also, document the use of ipmidriver_mutex
and tighten it's span some by incrementing the BMC's usecount in
the BMC find routines and not later.  This will be important for
future changes where a long mutex hold area will complicate things.

Signed-off-by: Corey Minyard <cminyard@mvista.com>