sun/cassini: mark cas_resume() as __maybe_unused

In certain configurations without power management support, gcc report
the following warning:

drivers/net/ethernet/sun/cassini.c:5206:12: warning:
 'cas_resume' defined but not used [-Wunused-function]
 5206 | static int cas_resume(struct device *dev_d)
      |            ^~~~~~~~~~

Mark cas_resume() as __maybe_unused to make it clear.

Fixes: f193f4eb ("sun/cassini: use generic power management")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/sun/cassini.c

@@ -5203,7 +5203,7 @@ static int __maybe_unused cas_suspend(struct device *dev_d)
 	return 0;
 }
 
-static int cas_resume(struct device *dev_d)
+static int __maybe_unused cas_resume(struct device *dev_d)
 {
 	struct net_device *dev = dev_get_drvdata(dev_d);
 	struct cas *cp = netdev_priv(dev);