Currently when mac80211 asks to change the interface
type, we will accept it for both the BSS and PAN
contexts. This is not terribly complicated today,
but with the addition of the P2P Device abstraction
the PAN context handling will get more complex, so
restrict mac_change_interface to the BSS context.

Also fix a small locking issue and use is_active
instead of the vif pointer to check if the other
context is activated, guarding exclusive interface
types on the BSS context (IBSS) against the PAN
context being used for something else.
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>