s390/qeth: use correct return type for hard_start_xmit()

ndo_start_xmit() expects us to return netdev_tx_t here...

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