mac80211: fix managed mode channel context use

My commit f2d9d270c15ae0139b54a7e7466d738327e97e03
("mac80211: support VHT association") introduced a
very stupid bug: the loop to downgrade the channel
width never attempted to actually use it again so
it would downgrade all the way to 20_NOHT. Fix it.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>