cxgb3: fix link flap

The driver is expected to report that the link is up
when the phy Rx signal is established and the mac
has not detected a link fault.
The code is however broken, the driver does not check the link fault
status when the phy link status changes.
The link fault status being checked within a short period of time,
it leads to link up/link down events.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>