netxen: implement pci driver shutdown

Implement pci driver shutdown functionality, this helps
quiescing all PCI transaction before chipset is reset.

Amit Kumar Salecha <amit@qlogic.com>
Signed-off-by: Dhananjay Phadke <dhananjay@netxen.com>
Signed-off-by: David S. Miller <davem@davemloft.net>