net: pch_gbe: convert to hw_features

This also fixes bug in xmit path, where TX checksum offload state was used
instead of skb->ip_summed to decide if the offload was needed.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>