gianfar: prevent buggy hw rx vlan tagging

On some buggy chips, "vlan tag present" flag is set which causes packet
loss. Fix this by checking if rx vlan accel is enabled in features.

Reported-by: Michael Guntsche <mguntsche@gmail.com>
Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>