[iwm] Use vap->iv_myaddr instead of ic->ic_macaddr when vap != NULL.

ic_macaddr is only used for the initial mac address provided by NVM. We should
rather use vap->iv_myaddr when vap != NULL, to allow the MAC address
to be changed later with ifconfig(8).

Submitted by: Imre Vadasz <imre@vdsz.com>
Reviewed by: avos
Approved by: re (gjb)
Obtained from: DragonflyBSD git 4aee7a78275676d22d14c04177bd0c9377d91478
Differential Revision: https://reviews.freebsd.org/D6743