cfg80211: disallow setting channel on WDS interfaces

If it worked (Felix says it doesn't right now), the
typical use-case for WDS interfaces would be to be
slaved to AP mode interfaces. Therefore, it isn't
necessary to set the channel on WDS interfaces. As
they don't support powersave or anything like that,
they also couldn't use a different channel anyway.

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