s390/qeth: rework RX/TX checksum offload

Rework the RX/TX checksum offloading command sequence to use
the provided function call back mechanims to return card
data to the device driver.

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