s390/qeth: consolidate skb RX processing in L3 driver

Use napi_gro_receive() to pass up all types of packets that a L3 device
may receive.
1) For proper L2 packets received by the IQD sniffer, this is the
   obvious thing to do.
2) For af_iucv (which doesn't provide a GRO assist), the GRO code will
   transparently fall back to netif_receive_skb(). So there's no need to
   special-case this traffic in our code.

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