tlan: proper shared IRQ support

Handle shared IRQ correctly. If IRQ is shared, it typically will show up
as an IRQ with an empty status field. So check in driver and handle it
without crapping out with invalid interrupt message.

Compile tested only.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>