sky2: check for more work before leaving NAPI

This patch avoids generating another IRQ if more packets
arrive while in the NAPI poll routine. Before marking device as
finished, it rechecks that the status ring is empty.

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