ipmi: Rework BMC registration

There was a certain error case where the BMC wouldn't be deregistered
like it should be.  Rework the BMC registration to make calling
ipmi_bmc_unregister() ok even if it's not registered and to clean up
the error handling for ipmi_bmc_register().

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