[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: NJean-Christophe Dubois <jdubois@mc.com>
Acked-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: NPaul Mackerras <paulus@samba.org>

arch/powerpc/sysdev/dcr.c

@@ -129,7 +129,7 @@ void dcr_unmap(dcr_host_t host, unsigned int dcr_n, unsigned int dcr_c)
 
 	if (h.token == NULL)
 		return;
-	h.token -= dcr_n * h.stride;
+	h.token += dcr_n * h.stride;
 	iounmap(h.token);
 	h.token = NULL;
 }