cxgb4: Add VF Link state support

Use ndo_set_vf_link_state to control the link states associated
with the virtual interfaces.

Signed-off-by: Arjun Vynipadath <arjun@chelsio.com>
Signed-off-by: Vishal Kulkarni <vishal@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>