sky2: fix shutdown synchronization

The logic in sky2_down was incorrect. Receiver could report status
after rx_stop was called.

The steps need to be:
   * stop new frames from being transmitted
   * shut off transmit/receive logic
   * synchronize with NAPI to process status info about transmitter
     and receiver

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