via-velocity: Change DMA_LENGTH_DEF (from the VIA driver)

The VIA driver has changed the default for the DMA_LENGTH_DEF parameter.
Together with adaptive interrupt supression and NAPI support, this
improves performance quite a bit

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