net: stmmac: platform: fix build error with !CONFIG_PM_SLEEP

commit 7ec05a60 upstream.

Get rid of the CONFIG_PM_SLEEP ifdefery to fix the build error
and use __maybe_unused for the suspend()/resume() hooks to avoid
build warning:

drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c:769:21:
 error: 'stmmac_runtime_suspend' undeclared here (not in a function); did you mean 'stmmac_suspend'?
  769 |  SET_RUNTIME_PM_OPS(stmmac_runtime_suspend, stmmac_runtime_resume, NULL)
      |                     ^~~~~~~~~~~~~~~~~~~~~~
./include/linux/pm.h:342:21: note: in definition of macro 'SET_RUNTIME_PM_OPS'
  342 |  .runtime_suspend = suspend_fn, \
      |                     ^~~~~~~~~~
drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c:769:45:
 error: 'stmmac_runtime_resume' undeclared here (not in a function)
  769 |  SET_RUNTIME_PM_OPS(stmmac_runtime_suspend, stmmac_runtime_resume, NULL)
      |                                             ^~~~~~~~~~~~~~~~~~~~~
./include/linux/pm.h:343:20: note: in definition of macro 'SET_RUNTIME_PM_OPS'
  343 |  .runtime_resume = resume_fn, \
      |                    ^~~~~~~~~

Fixes: 5ec55823

 ("net: stmmac: add clocks management for gmac driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Yongxin Liu <yongxin.liu@windriver.com>