igb: mark PM functions as __maybe_unused

The new wake function is only used by the suspend/resume handlers that
are defined in inside of an #ifdef, which can cause this harmless
warning:

drivers/net/ethernet/intel/igb/igb_main.c:7988:13: warning: 'igb_deliver_wake_packet' defined but not used [-Wunused-function]

Removing the #ifdef, instead using a __maybe_unused annotation
simplifies the code and avoids the warning.

Fixes: b90fa876 ("igb: Enable reading of wake up packet")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>