cfg80211: warn when wiphy_apply_custom_regulatory() does nothing

Device drivers using wiphy_apply_custom_regulatory() want some
regulatory settings applied to their wiphy, if no bands were
configured on the wiphy then something went wrong.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>