net/mlx5: cmd: Fix memset with byte count warning

Fix sparse warning:
drivers/net/ethernet/mellanox/mlx5/core/cmd.c:1949:15:
warning: memset with byte count of 271720

mlx5_cmd_stats array is too big to be held inline in mlx5_cmd.
Allocate it separately.

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