net: ethernet: ti: Use PTR_ERR_OR_ZERO() to simplify code

Fixes coccicheck warning:

drivers/net/ethernet/ti/am65-cpts.c:1017:1-3: WARNING: PTR_ERR_OR_ZERO can be used

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Samuel Zou <zou_wei@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>