sun/niu: add __maybe_unused attribute to PM functions

The upgraded .suspend() and .resume() throw
"defined but not used [-Wunused-function]" warning for certain
configurations.

Mark them with "__maybe_unused" attribute.

Compile-tested only.

Fixes: b0db0cc2 ("sun/niu: use generic power management")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Vaibhav Gupta <vaibhavgupta40@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sun/niu.c

@@ -9873,7 +9873,7 @@ static void niu_pci_remove_one(struct pci_dev *pdev)
 	}
 }
 
-static int niu_suspend(struct device *dev_d)
+static int __maybe_unused niu_suspend(struct device *dev_d)
 {
 	struct net_device *dev = dev_get_drvdata(dev_d);
 	struct niu *np = netdev_priv(dev);
@@ -9900,7 +9900,7 @@ static int niu_suspend(struct device *dev_d)
 	return 0;
 }
 
-static int niu_resume(struct device *dev_d)
+static int __maybe_unused niu_resume(struct device *dev_d)
 {
 	struct net_device *dev = dev_get_drvdata(dev_d);
 	struct niu *np = netdev_priv(dev);