net/mlx5e: Reduce number of heap allocated buffers for update stats

Allocating buffers on the heap every 200ms is something we should avoid,
let's use buffers located on the stack instead.

Signed-off-by: Gal Pressman <galp@mellanox.com>
Reviewed-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Cc: kernel-team@fb.com