boot_loader_openfirmware: Fix trace output

Update the variable name.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@39430 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/system/boot/platform/openfirmware/arch/ppc/mmu.cpp

@@ -645,7 +645,7 @@ arch_set_callback(void)
 		puts("set-callback failed!");
 		return B_ERROR;
 	}
-	//printf("old callback = %p\n", old);
+	//printf("old callback = %p\n", oldCallback);
 
 	return B_OK;
 }