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.
This should also help porting code to other OSs.
Sponsored by: The FreeBSD Foundation
MFC after: 1 week