skge: handle zero address at open

Some motherboards are broken and have no address set. Failing at probe time
prevents the device from ever being used (like to download a fixed BIOS). Instead
warn on probe and check again when device is brought up. That way the address
can be set.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>