cfg80211: allow only the core to request to update the world regdom

It seems some out of tree drivers were using a regulatory_hint("00")
to trigger off the wiphy regulatory notifier, for those cases just
setting the WIPHY_FLAG_CUSTOM_REGULATORY would suffice to call
the reg_notifier() for a world regulatory domain update. If drivers
find other needs for calling the reg_notifier() a proper implemenation
is preferred.

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>