via-velocity: Add ethtool interrupt coalescing support

(Partially from the upstream VIA driver). Tweaking the number of
frames-per-interrupt and timer-until-interrupt can reduce the amount of
CPU work quite a lot.

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