LinuxKPI: 802.11: use net80211 IEEE80211_HTCAP_* definitions

Rather than using the values and leaving net80211 names in a comment
define the LinuxKPI IEEE80211_HT_CAP_* to the net80211 IEEE80211_HTCAP_*
names. That way errors like the one fixed in 3e0915b7b685 are less
likely to happen.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days