mac80211: clean up mesh HT operation

ieee80211_ht_cap_ie_to_sta_ht_cap() will clean up the
ht_supported flag and station bandwidth field for us
if the peer beacon doesn't have an HT capability element
(is operating as non-HT).

Also, we don't really need a special station ch_width
member to track the station operating mode any more so use
sta.bandwidth instead.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>