cfg80211: fix regulatory code const

A few pointers and structures in the regulatory code are const,
but because it wasn't done properly a whole bunch of bogus
casts were needed to compile without warning. Mark everything
const properly to avoid that kind of junk code.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>