net/mlx5e: Use dynamic per-channel allocations in stats

Make stats array an array of pointer. This patch comes in to prepare for
the next patch where allocations of the stats are to be performed
dynamically on first usage.

Signed-off-by: Lama Kayal <lkayal@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>