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: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>