mac80211: fix HT40 connections

My commit 4bf88530
("mac80211: convert to channel definition struct")
accidentally broke HT40 connections due to swapped
channel flag checks -- fix that.

Reported-by: Sujith Manoharan <sujith@msujith.org>
Tested-by: Sujith Manoharan <sujith@msujith.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>