s390/qeth: align csum offload with TSO control logic

csum offload and TSO have similar programming requirements. The TSO code
was reworked with commit "s390/qeth: enhance TSO control sequence",
adjust the csum control flow accordingly. Primarily this means replacing
custom helpers with more generic infrastructure.

Also, change the LP2LP check so that it warns on TX offload (not RX).
This is where reduced csum capability actually matters.

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