sky2: manage irq better on single port card

Most sky2 hardware only has a single port, although some variations of the
chip support two interfaces.  For the single port case, use the standard
Ethernet driver convention of allocating IRQ when device is brought up
rather than at probe time.

Also, change the error handling of dual port cards so that if second
port can not be brought up, then just fail. No point in continuing, since
the failure is most certainly because of out of memory.

The dual port sky2 device has a single irq and a single status ring,
therefore it has a single NAPI object shared by both ports.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>