e1000e: fix unused-function warning

The CONFIG_PM_SLEEP #ifdef checks in this file are inconsistent,
leading to a warning about sometimes unused function:

drivers/net/ethernet/intel/e1000e/netdev.c:137:13: error: unused function 'e1000e_check_me' [-Werror,-Wunused-function]

Rather than adding more #ifdefs, just remove them completely
and mark the PM functions as __maybe_unused to let the compiler
work it out on it own.

Fixes: e086ba2f

 ("e1000e: disable s0ix entry and exit flows for ME systems")
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>