net: bcmgenet: correct per TX/RX ring statistics

The change to track net_device_stats per ring to better support SMP
missed updating the rx_dropped member.

The ndo_get_stats method is also needed to combine the results for
ethtool statistics (-S) before filling in the ethtool structure.

Fixes: 37a30b43 ("net: bcmgenet: Track per TX/RX rings statistics")
Signed-off-by: Doug Berger <opendmb@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>