[POWERPC] Fix dcr_unmap()

Fix a bug in dcr_unmap().

At unmap time the DCR offset need to be added instead of substracted.

Signed-off-by: Jean-Christophe Dubois <jdubois@mc.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>