net/mlx5e: Add XDP_TX completions statistics

Add per-ring and global ethtool counters for XDP_TX completions.
This helps us monitor and analyze XDP_TX flow performance.

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