net/mlx5e: Changed naming convention of tx queues in ethtool stats

Instead of the pair (channel, tc), we now use a single number that
goes over all tx queues of a TC, for all TCs.

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