net: aquantia: fix error handling in aq_ptp_poll

Fix currenty ignored returned error by properly checking *err* after
calling aq_nic->aq_hw_ops->hw_ring_hwts_rx_fill().

Addresses-Coverity-ID: 1487357 ("Unused value")
Fixes: 04a18399 ("net: aquantia: implement data PTP datapath")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Igor Russkikh <irusskikh@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>