[POWERPC] PS3: Fix storage probe logic

Fix the PS3 storage probe logic to properly find device regions on cold
startup.

 o Change the storage probe event mask from notify_device_ready
   to notify_region_update.
 o Improve the storage probe error handling.
 o Change ps3_storage_wait_for_device() to use a temporary variable to hold
   the buffer address.

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