net/mlx5: avoid build warning for uniprocessor

Building the driver with CONFIG_SMP disabled results in a harmless
warning:

ethernet/mellanox/mlx5/core/main.c: In function 'mlx5_irq_set_affinity_hint':
ethernet/mellanox/mlx5/core/main.c:615:6: error: unused variable 'irq' [-Werror=unused-variable]

It's better to express the conditional compilation using IS_ENABLED()
here, as that lets the compiler see what the intented use for the variable
is, and that it can be silently discarded.

Fixes: b665d98edc9a ("net/mlx5: Tolerate irq_set_affinity_hint() failures")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/mellanox/mlx5/core/main.c

@@ -621,10 +621,9 @@ static int mlx5_irq_set_affinity_hint(struct mlx5_core_dev *mdev, int i)
 	cpumask_set_cpu(cpumask_local_spread(i, priv->numa_node),
 			priv->irq_info[i].mask);
 
-#ifdef CONFIG_SMP
-	if (irq_set_affinity_hint(irq, priv->irq_info[i].mask))
+	if (IS_ENABLED(CONFIG_SMP) &&
+	    irq_set_affinity_hint(irq, priv->irq_info[i].mask))
 		mlx5_core_warn(mdev, "irq_set_affinity_hint failed, irq 0x%.4x", irq);
-#endif
 
 	return 0;
 }