net: velocity: Rename vptr->dev to vptr->netdev

Improve the clarity of the code in preparation for converting the
dma functions to generic versions, which require a struct device *.

This makes it possible to store a 'struct device *dev' in the
velocity_info structure.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>