phy:phy-lantiq-rcu-usb2: Use PTR_ERR_OR_ZERO to replace the open coded version

PTR_ERR_OR_ZERO has implemented the if(IS_ERR(...)) + PTR_ERR, So
just replace them rather than duplicating its implement.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>