net/mlx5: Fix indexing of mlx5_irq

After the cited patch, mlx5_irq xarray index can be different then
mlx5_irq MSIX table index.
Fix it by storing both mlx5_irq xarray index and MSIX table index.

Fixes: 3354822c

 ("net/mlx5: Use dynamic msix vectors allocation")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>