net/mlx5: Refcount mlx5_irq with integer

Currently, all access to mlx5 IRQs are done undere a lock. Hance, there
isn't a reason to have kref in struct mlx5_irq.
Switch it to integer.

Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>