ipmi: only register one si per bmc

Only register one si per bmc.  Use any user-provided devices first,
followed by the first device with an irq, followed by the first device
discovered.

Signed-off-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>