[POWERPC] PS3: Refactor ps3_repository_find_device()

PS3: Refactor ps3_repository_find_device() to use the existing
ps3_repository_read_bus_id() routine.

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>