ksz884x: mark pcidev_suspend() as __maybe_unused

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

drivers/net/ethernet/micrel/ksz884x.c:7182:12: warning:
 'pcidev_suspend' defined but not used [-Wunused-function]
 7182 | static int pcidev_suspend(struct device *dev_d)
      |            ^~~~~~~~~~~~~~

Mark pcidev_suspend() as __maybe_unused to make it clear.

Fixes: 64120615 ("ksz884x: use generic power management")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>