s390/qeth: remove skb_is_nonlinear() check on IQD

qeth doesn't advertise NETIF_F_SG for L3 IQDs. So trust the stack to
not hand us any nonlinear skbs, and remove an always-true condition.

With the fact that data_offset < 0 is no longer possible on IQDs,
apply a small cleanup to subsequent code.

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>