s390/qeth: support GRO flush timer

Switch to napi_complete_done(), and thus enable delayed GRO flushing.
The timeout is configured via /sys/class/net/<if>/gro_flush_timeout.

Default timeout is 0, so no change in behaviour.

Signed-off-by: Julian Wiedmann <jwi@linux.vnet.ibm.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>