mac80211: Update conf_is_ht() to work properly with 5/10MHz channels

The channels with 5/10MHz bandwidth are not HT. We have to
reflect this in conf_is_ht() function which returns whether the
particular channel is HT or not.

Signed-off-by: Rostislav Lisovy <rostislav.lisovy@fel.cvut.cz>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>