[PATCH] powerpc: Fix g5 DART init

The patch enabling the new G5's with U4 broke initialization of the DART
driver, causing it to trigger a BUG_ON for a case that is actually
valid. This patch fixes it:

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>