[POWERPC] PS3: Fix bootwrapper hang bug

Fix a bug in the lv1_get_repository_node_value() routine of the PS3
bootwrapper.  Changes in the PS3 system firmware 2.20 cause this bug
to hang the system when branching from the bootwrapper to the kernel
_start.

Since the video system has not yet been enabled at the time
the bug is hit, the system hangs with a blank screen.  Earlier
firmwares don't cause such a catastrophic failure, and so this
bug went undetected.

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