mac80211: restricted automatic channel selection

Add a new config option "channels" for mac80211 wifi devices. It's only
valid if automatic channel selection is used and restricts the channel
selection to one of the given channels.

config wifi-device
	list channels	1
	list channels	6
	list channels	11

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@47427 3c298f89-4303-0410-b956-a3cf2f4a3e73