mac80211: Fix HT channel selection

HT management is done differently for AP and STA modes, unify
to just the ->config() callback since HT is fundamentally a
PHY property and cannot be per-BSS.

Rename enum nl80211_sec_chan_offset as nl80211_channel_type to denote
the channel type ( NO_HT, HT20, HT40+, HT40- ).

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>