[POWERPC] Split out the ioid fetching/checking logic

Split out the ioid fetching and checking logic so we can use it elsewhere
in a subsequent patch.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>