mac80211: prepare TDLS mgmt code for channel-switch templates

Split the data-generating from the Tx-sending functionality, as we do
not want to send templates to the lower driver. Also add an optional
chandef argument to the data-generating portion. It will be used for
channel-switch templates.

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>