via-velocity: Implement NAPI support

This patch adds NAPI support for VIA velocity. The new velocity_poll
function also pairs tx/rx handling twice which improves perforamance on
some workloads (e.g., netperf UDP_STREAM) significantly (that part is
from the VIA driver).

Signed-off-by: Simon Kagstrom <simon.kagstrom@netinsight.net>
Signed-off-by: David S. Miller <davem@davemloft.net>