ipmi/of: Don't use unavailable interfaces

If an IPMI controller is used by the firmware and as such marked with
a reserved status, we shouldn't use it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>