net80211: crypto: enhance comment about IEEE80211_RX_F_MMIC_STRIP

net80211 crypto currently re-uses the M[ichael]MIC flag for MIC as well
at least in CCMP. This is a bit confusing so at least try to improve
the comment that it becomes more obvious.
In the long-term we may want to just add a MIC flag as well?

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D48978