cfg80211: Purge frame registrations on iftype change

Currently frame registrations are not purged, even when changing the
interface type.  This can lead to potentially weird situations where
frames possibly not allowed on a given interface type remain registered
due to the type switching happening after registration.

The kernel currently relies on userspace apps to actually purge the
registrations themselves, this is not something that the kernel should
rely on.

Add a call to cfg80211_mlme_purge_registrations() to forcefully remove
any registrations left over prior to switching the iftype.

Cc: stable@vger.kernel.org
Signed-off-by: Denis Kenzior <denkenz@gmail.com>
Link: https://lore.kernel.org/r/20190828211110.15005-1-denkenz@gmail.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>