mac80211: add HT conf helpers

In HT capable drivers you often need to check if you
are currently using HT20 or HT40. This adds a few small
helpers to let drivers figure that out.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>