atl1: update statistics code

As Ben Hutchings pointed out for the stats in alx, some
hardware-specific stats aren't matched to the right net_device_stats
field. Also fix the collision field and include errors in the total
number of RX/TX packets. Add a rx_dropped field and use it where
netdev->stats was modified directly out of the stats update function.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>