mac80211: disconnect when user changes channel

If we do not disconnect when a channel switch is requested,
we end up eventually detection beacon loss from the AP and
then disconnecting, without ever really telling the AP, so
we might just as well disconnect right away.

Additionally, this fixes a problem with iwlwifi where the
driver will clear some internal state on channel changes
like this and then get confused when we actually go clear
that state from mac80211.

It may look like this patch drops the no-IBSS check, but
that is already handled by cfg80211 in the wext handler it
provides for IBSS (cfg80211_ibss_wext_siwfreq).

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