[POWERPC] EEH: wait for slot status

Modify routine that returns PCI slot status to wait for slot status
to become available. This is needed, as slots that are in some remote
card cage may go offline for extended periods of time. New users for
this routine in following patches.

Signed-off-by: Linas Vepstas <linas@austin.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>