cxgb4: depend on firmware event for link status

Depend on the firmware sending us link status changes,
rather than assuming that the link goes down upon L1
configuration.

Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>