forcedeth: napi schedule lock fix

This patch fixes a potential race condition between scheduling napi and
completing napi poll. The call to netif_rx_schedule should be under
protection of the lock (as is the completion), otherwise, interrupts
could be masked off.

Signed-off-by: Ayaz Abdulla <aabdulla@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>