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: NJulian Wiedmann <jwi@linux.vnet.ibm.com>
Acked-by: NUrsula Braun <ubraun@linux.vnet.ibm.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>