cfg80211: move core reg_notfier() check to source

Drivers that set the WIPHY_FLAG_CUSTOM_REGULATORY skip
the core world regulatory domain updates, but do want
their reg_notifier() called. Move the check for this
closer to the source of the check that detected skipped
was required and while at it add a helper for the notifier
calling. This has no functional changes. This brings together
the place where we call the reg_notifier() will be called.

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