ipmi: Add a reference from BMC devices to their interfaces

In an upcoming change, we'll want to grab a reference to the ipmi_smi_t
from a struct bmc_device. This change adds a pointer to allow this.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>

Reworked to support multiple interfaces on a BMC.

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