mac80211: properly track HT operation_mode

When we disassociate, we set the channel to non-HT which
obviously invalidates any ht_operation_mode setting. But
when we then associate with the next AP again, we might
still have the ht_operation_mode from the previous AP
cached and fail to configure the hardware with the new
(but unchanged) operation mode. This patch fixes it by
separately tracking whether our cache is valid.

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