cxgb4: Do not set net_device::dev_id to VI index

net_device::dev_id should not be used merely to indicate a VI index,
as it affects the way the local part of IPv6 addresses is normally
generated.

This field was intended for use where multiple devices may share a
single assigned MAC address and need to have different IPv6 addresses.
T4 VIs each have their own MAC address.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Acked-by: Dimitris Michailidis <dm@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>