cxgb4: fix to bring link down after adapter crash

Use PORT_REG for T4 and T5_PORT_REG for > T4 to write to correct
register to bring down link during shutdown after adapter crash.

Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>