bnxt(4): Fix bugs in WOL support.

Before this change driver reported IFCAP_WOL_MAGIC enabled, but not
supported. It caused errors on some SIOCSIFCAP calls. Instead
report the support if hardware supports WOL, and enabled status if
it has such filter installed on boot.

Also bnxt_wol_config() should check WOL status in if_getcapenable(),
not in if_getcapabilities() to get current one.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.