s390/qeth: enable only required csum offload features

Current code attempts to enable all advertised HW csum offload features.
Future-proof this by enabling only those features that we actually use.

Also, the IPv4 header csum feature is only needed for TX on L3 devices.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>