net/mlx5e: Merge per priority stats groups

Merge the per priority traffic and pfc groups into one group, because
both groups share the same update_stats() callback which will be
introduced in the upcoming patch.

Signed-off-by: Kamal Heib <kamalh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>