ipmi/ipmi_powernv: remove outdated todo in powernv IPMI driver

Since the IPMI core now queries device IDs dynamically, we no longer
need this todo for implementing this in the powernv driver.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Corey Minyard <cminyard@mvista.com>