enc28j60: use netif_rx_ni() to deliver RX packets

The enc28j60 driver reads incoming packets in the process (workqueue) context,
not in a tasklet or the interrupt context.  Thus, we should use netif_rx_ni()
to deliver those packets to the networking layer, instead of netif_rx(). This
way incoming packets don't wait in the incoming queue for the next IRQ to be
serviced.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: David S. Miller <davem@davemloft.net>