cxgb4: keep carrier off before registering netdev

Mark carrier off before registering netdev to ensure that vlan device
picks up the correct state of the carrier

Signed-off-by: Surendra Mobiya <surendra@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>