net: bcmgenet: Track per TX/RX rings statistics

__bcmgenet_tx_reclaim() is currently summing TX bytes/packets in a way
that is not SMP friendly, mutliples CPUs could run
__bcmgenet_tx_reclaim() independently and still update stats->tx_bytes
and stats->tx_packets, cloberring the other CPUs statistics.

Fix this by tracking per RX and TX rings the number of bytes, packets,
dropped and errors statistics, and provide a bcmgenet_get_stats()
function which aggregates everything and returns a consistent output.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>