net/mlx5: Allocate command stats with xarray

Command stats is an array with more than 2K entries, which amounts to
~180KB. This is way more than actually needed, as only ~190 entries
are being used.

Therefore, replace the array with xarray.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>