ice: mark PM functions as __maybe_unused

In certain configurations without power management support, the
following warnings happen:

drivers/net/ethernet/intel/ice/ice_main.c:4214:12: warning:
 'ice_resume' defined but not used [-Wunused-function]
 4214 | static int ice_resume(struct device *dev)
      |            ^~~~~~~~~~
drivers/net/ethernet/intel/ice/ice_main.c:4150:12: warning:
 'ice_suspend' defined but not used [-Wunused-function]
 4150 | static int ice_suspend(struct device *dev)
      |            ^~~~~~~~~~~

Mark these functions as __maybe_unused to make it clear to the
compiler that this is going to happen based on the configuration,
which is the standard for these types of functions.

Fixes: 769c500d ("ice: Add advanced power mgmt for WoL")
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Tested-by: NAndrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: NTony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/ice/ice_main.c

@@ -4467,7 +4467,7 @@ static int ice_reinit_interrupt_scheme(struct ice_pf *pf)
  * Power Management callback to quiesce the device and prepare
  * for D3 transition.
  */
-static int ice_suspend(struct device *dev)
+static int __maybe_unused ice_suspend(struct device *dev)
 {
 	struct pci_dev *pdev = to_pci_dev(dev);
 	struct ice_pf *pf;
@@ -4531,7 +4531,7 @@ static int ice_suspend(struct device *dev)
  * ice_resume - PM callback for waking up from D3
  * @dev: generic device information structure
  */
-static int ice_resume(struct device *dev)
+static int __maybe_unused ice_resume(struct device *dev)
 {
 	struct pci_dev *pdev = to_pci_dev(dev);
 	enum ice_reset_req reset_type;