cxgb4: fix reported state of interfaces without link

Currently tools like ip and ifconfig report incorrect state for cxgb4
interfaces that are up but do not have link and do so until first link
establishment.  This is because the initial netif_carrier_off call is
before register_netdev and it needs to be after to be fully effective.
Fix this by moving netif_carrier_off into .ndo_open.

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