netxen: async link event handling

Add support for asynchronous events from firmware,
received over one of the rx rings.

Add support for event based phy interrupts, enhanced links
status reporting from firmware.

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