cxgb4: keep interrupts available when the ports are brought down

The PF driver needs to remain alert while its ports are down to service
requests or errors from virtual functions or FW so keep interrupts and
queues available when the ports are brought down.  The change makes
open_device_map unnecessary so remove it.

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