net: rocker: Add basic netdev counters - v2

Add packet and byte counters for RX and TX paths.

$ ifconfig eth1
eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet6 fe80::5054:ff:fe12:3501  prefixlen 64  scopeid 0x20<link>
        ether 52:54:00:12:35:01  txqueuelen 1000  (Ethernet)
        RX packets 63  bytes 15813 (15.4 KiB)
        RX errors 1  dropped 0  overruns 0  frame 0
        TX packets 79  bytes 17991 (17.5 KiB)
        TX errors 7  dropped 0 overruns 0  carrier 0  collisions 0

Rx / Tx errors tested by injecting faults in qemu's hardware model for Rocker.

v2:
- moved counter locations to avoid potential use after free per Florian's comment

Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Scott Feldman <sfeldma@gmail.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Scott Feldman <sfeldma@gmail.com>
Acked-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>