pcnet32: Mark PM functions as __maybe_unused

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

../drivers/net/ethernet/amd/pcnet32.c:2928:12: warning:
'pcnet32_pm_resume' defined but not used [-Wunused-function]
 2928 | static int pcnet32_pm_resume(struct device *device_d)
      |            ^~~~~~~~~~~~~~~~~
../drivers/net/ethernet/amd/pcnet32.c:2916:12: warning:
'pcnet32_pm_suspend' defined but not used [-Wunused-function]
 2916 | static int pcnet32_pm_suspend(struct device *device_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: a86688fb

 ("pcnet32: Convert to generic power management")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>