net80211: consistently use the IEEE80211_M_ memory related options

Replace a malloc() by IEEE80211_MALLOC().
For malloc flags even in the local ieee80211_freebsd.c there was a mix
of both versions M_ and IEEE80211_M_.
Consistently use the IEEE80211_M_ malloc options everywhere.
If the field is changed for malloc, it'll also be changed for the
other accessor functions taking a "how" field to avoid any confusion.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D36249