amd8111e: Mark PM functions as __maybe_unused

In certain configurations without power management support, the
following warnings happen:

../drivers/net/ethernet/amd/amd8111e.c:1623:12: warning:
'amd8111e_resume' defined but not used [-Wunused-function]
 1623 | static int amd8111e_resume(struct device *dev_d)
      |            ^~~~~~~~~~~~~~~
../drivers/net/ethernet/amd/amd8111e.c:1584:12: warning:
'amd8111e_suspend' defined but not used [-Wunused-function]
 1584 | static int amd8111e_suspend(struct device *dev_d)
      |            ^~~~~~~~~~~~~~~~

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: 2caf751f

 ("amd8111e: Convert to generic power management")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>