[POWERPC] cell/PS3: Ignore storage devices that are still being probed

On PS3, A storage device may show up in the repository before the hypervisor
has finished probing:
  - If its type is not yet known, it shows up as PS3_DEV_TYPE_STOR_DUMMY,
  - If its regions are being probed, it shows up as having zero regions.
If any of these happen, consider the device not yet present.  The storage
probe thread will retry later.

This fixes the timing-dependent problem where a kernel booted from FLASH ROM
sometimes cannot find the hard disk.

Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Acked-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>