mac80211: Disallow changing chan type on monitor when CHAN_MODE_FIXED

If you add a monitor interface in parallel to a normal interface
mac80211 will let you to change the channel type on the monitor
interface even if you are connected. Add an explicit check to
disallow this.

Signed-off-by: Pontus Fuchs <pontus.fuchs@gmail.com>
[fix typo in commit log, use sdata instead of netdev]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>