e1000e: Mark e1000e_pm_prepare() as __maybe_unused

The function e1000e_pm_prepare() may have no callers depending
on configuration, so it must be marked __maybe_unused to avoid
harmless warning:

drivers/net/ethernet/intel/e1000e/netdev.c:6926:12:
 warning: 'e1000e_pm_prepare' defined but not used [-Wunused-function]
 6926 | static int e1000e_pm_prepare(struct device *dev)
      |            ^~~~~~~~~~~~~~~~~

Fixes: ccf8b94 ("e1000e: Leverage direct_complete to speed up s2ram")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/e1000e/netdev.c

@@ -6919,7 +6919,7 @@ static int __e1000_resume(struct pci_dev *pdev)
 	return 0;
 }
 
-static int e1000e_pm_prepare(struct device *dev)
+static __maybe_unused int e1000e_pm_prepare(struct device *dev)
 {
 	return pm_runtime_suspended(dev) &&
 		pm_suspend_via_firmware();