parisc: pdt: Use PTR_ERR_OR_ZERO() to simplify code

Return PTR_ERR_OR_ZERO() instead of return 0 or PTR_ERR() to
simplify code.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Helge Deller <deller@gmx.de>