sun/cassini: mark cas_resume() as __maybe_unused

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

drivers/net/ethernet/sun/cassini.c:5206:12: warning:
 'cas_resume' defined but not used [-Wunused-function]
 5206 | static int cas_resume(struct device *dev_d)
      |            ^~~~~~~~~~

Mark cas_resume() as __maybe_unused to make it clear.

Fixes: f193f4eb

 ("sun/cassini: 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>