cfg/mac80211: define TDLS channel switch feature bit

Define some related TDLS protocol constants and advertise channel switch
support in the extended-capabilities IE when the feature bit is defined.

Actually supporting TDLS channel-switching also requires support for
some new nl80211 commands, to be introduced by future patches.

Signed-off-by: Arik Nemtsov <arikx.nemtsov@intel.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>