net/mlx5e: IPoIB, Add ndo stats support for IPoIB netdevices

Expose RX and TX counters by implementing ndo_get_stats64 operation for
both parent devices.
After this change, all the relevant statistics can be retrieved using
ifconfig.

Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>