net/mlx5: fix constant array size

net/mlx5: fix constant array size

Before this commit the PMD used:
   const int elt_n = 8
   const int *stack[elt_n];

In Windows clang compiler complains:
net/mlx5/mlx5_flow.c:215:19: error: variable length array folded
to constant array as an extension [-Werror,-Wgnu-folding-constant]

Fix it by using a constant macro definition instead of a variable:
   #define MLX5_RSS_EXP_ELT_N 8
   const int *stack[MLX5_RSS_EXP_ELT_N];

Fixes: c7870bfe09dc ("ethdev: move RSS expansion code to mlx5 driver")
Cc: stable@dpdk.org
Signed-off-by: Tal Shnaiderman <talshn@nvidia.com>
Signed-off-by: Ophir Munk <ophirmu@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>