ieee80211_rate2plcp() and ieee80211_rate2phytype() are both pre-11n
routines and thus assert if one passes in a rate code with the
high bit set.

Since the high bit can indicate either IEEE80211_RATE_BASIC or
IEEE80211_RATE_MCS, it's up to the caller to determine whether
the rate is 11n or not, and either mask out the BASIC bit, or
call a different function.

(Yes, this does mean that net80211 should grow 11n-aware rate2phytype()
and rate2plcp() functions..)

This may need to happen for the other drivers - it's currently only
done (now) for iwn(4) and bwi(4).

PR:		kern/181100