Update WOL support for newer em(4) devices.

Do not overwrite the contents of the WUC register, add E1000_WUC_PME_EN
to the register contents, leaving the default contents intact.

PR: 208343
Submitted by: Kaho Toshikazu <kaho@elam.kais.kyoto-u.ac.jp>
Reviewed by: jeffrey piper <jeffrey.e.pieper@intel.com>
Approved by: erj@
MFC after: 2 weeks