net/mlx5e: Add rx/tx bytes software counters

Sum up rx/tx bytes in software as we do for rx/tx packets, to be reported
in upcoming statistics fix.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>