LinuxKPI: 802.11: avoid using an mbuf tag for now

We are using an mbuf tag to carry the ni reference in the TX path
from the TX function past the taskq along with the mbuf.
Contrary to initial assumptions we only need the ni and no other data
so attach the ni to m->m_pkthdr.PH_loc.ptr avoiding the extra allcation.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days