mac80211: move spectrum management code out

Like the HT code, this doesn't depend on the STA-mode implementation
and can be handled entirely independently. There's only stub code
for now, but when it gets filled having it in its own file will be
beneficial.

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