net: stmmac: Only consider RX error when HW Timestamping is not enabled

Only consider that we have an error when HW Timestamping is not enabled
as this can give false positives due to the fact the RX Timestamping in
XGMAC and GMAC cores comes from context descriptors.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>