via-velocity: Re-enable transmit scatter-gather support

The velocity hardware can handle up to 7 memory segments. This can be
turned on and off via ethtool. The support was removed in commit

  83c98a8c

but is re-enabled and cleaned up here. It's off by default.

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