802.11: clean up/fix HT support

This patch cleans up a number of things:
 * the unusable definition of the HT capabilities/HT information
   information elements
 * variable names that are hard to understand
 * mac80211: move ieee80211_handle_ht to ht.c and remove the unused
             enable_ht parameter
 * mac80211: fix bug with MCS rate 32 in ieee80211_handle_ht
 * mac80211: fix bug with casting the result of ieee80211_bss_get_ie
             to an information element _contents_ rather than the
             whole element, add size checking (another out-of-bounds
             access bug fixed!)
 * mac80211: remove some unused return values in favour of BUG_ON
             checking
 * a few minor other things

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