net/mlx5: Use first online CPU instead of hard coded CPU

Hard coded CPU (0 in our case) might be offline. Hence, use the first
online CPU instead.

Fixes: f891b7cd ("net/mlx5: Enable single IRQ for PCI Function")
Signed-off-by: Shay Drory <shayd@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>