ftgmac100: Work around HW bug in runt frame detection

The HW incorrectly calculates the frame size without the vlan
tag and compares that against 64. It will thus flag 64-bytes
frames with a vlan tag as 60-bytes frames "runt" packets
which we'll then drop. Thus we end up dropping ARP packets
on vlan's ...

It does that whether vlan tag stripping is enabled or not.

This works around it by ignoring the "runt" error bit of the
frame has been vlan tagged and is at least 60 bytes.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>