cfg80211: avoid intersection when applying self-managed reg

The custom-reg handling function can currently only add flags to a given
channel. This results in stale flags being left applied. In some cases
a channel was disabled and even the orig_flags were changed to reflect
this.

Previously the API was designed for a single invocation before wiphy
registration, so this didn't matter. The previous approach doesn't scale
well to self-managed regulatory devices, particularly when a more
permissive regdom is applied after a restrictive one.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>