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: ccf8b940 ("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>