net: aquantia: fix return value check in aq_ptp_init()

Function ptp_clock_register() returns ERR_PTR() and never returns
NULL. The NULL test should be removed.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>