sky2: status ring race fix

The D-Link PCI-X board (and maybe others) can lie about status
ring entries. It seems it will update the register for last status
index before completing the DMA for the ring entry. To avoid reading
stale data, zap the old entry and check.

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