net: rocker: Add support for retrieving port level statistics

Add support for retrieving port level statistics from device.
Hook is added for ethtool's stats functionality. For example,

$ ethtool -S eth3
NIC statistics:
     rx_packets: 12
     rx_bytes: 2790
     rx_dropped: 0
     rx_errors: 0
     tx_packets: 8
     tx_bytes: 728
     tx_dropped: 0
     tx_errors: 0

Signed-off-by: David Ahern <dsahern@gmail.com>
Acked-by: Scott Feldman <sfeldma@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>