ieee1394: sbp2: Kconfig fix

We only support x86 and ppc, due to the use of bus_to_virt() and friends.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Ben Collins <bcollins@ubuntu.com>