cfg80211: set core regulatory updates on its own

This splits up the core regulatory update to be
set on its own helper. This should make it easier
to read exactly what type of requests should be
expected there. In this case its clear that
NL80211_REGDOM_SET_BY_CORE is only used by the
core for updating the world regulatory domain.
This is consistant with the nl80211.h documentation.

Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>
[add warning to default switch case to avoid compiler warning]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>