net: systemport: Track per TX ring statistics

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

Fix this by tracking per TX rings the number of bytes, packets,
dropped and errors statistics, and provide a bcm_sysport_get_nstats()
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>