powerpc: Fix compile with icwsx support

Due to a collision between NO_CONTEXT->MMU_NO_CONTEXT change and
Anton's patch.

Signed-off-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

arch/powerpc/mm/mmu_context_hash64.c

···
 	if (!mm->context.cop_lockp) {
 		__destroy_context(index);
 		subpage_prot_free(mm);
-		mm->context.id = NO_CONTEXT;
+		mm->context.id = MMU_NO_CONTEXT;
 		return -ENOMEM;
 	}
 	spin_lock_init(mm->context.cop_lockp);