qeth layer 2 and layer 3 common feature handling

This patch introduces a common set of fix_features and set_features
functions for layer 2 and layer 3. The RX, TX and TSO offload
functionality on the OSA card is enabled using ethtool at user's
request and not at device initialization as done before.

For layer 3 the RX checksum offloading is disabled at device
initialization time.

Signed-off-by: Thomas Richter <tmricht@linux.vnet.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>