net/mlx5e: Switch channels counters to use stats group API

Switch the channels counters to use the new stats group API.

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