net: stmmac: don't overwrite discard_frame status

[ Upstream commit 1b746ce8b397e58f9e40ce5c63b7198de6930482 ]

If we have error bits set, the discard_frame status will get overwritten
by checksum bit checks, which might set the status back to good one.
Fix by checking the COE status only if the frame is good.

Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>