net/mlx5e: Add VLAN offloads statistics

The following counters are now exposed through ethtool -S:
rx[i]_removed_vlan_packets (per channel)
rx_removed_vlan_packets
tx[i]_added_vlan_packets (per channel)
tx_added_vlan_packets

rx_removed_vlan_packets: The number of packets that had their
outer VLAN header stripped to the CQE by the hardware.
tx_added_vlan_packets: The number of packets that had their
outer VLAN header inserted by the hardware.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Reviewed-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>