mac80211: fix TX legacy rate reporting when tx_status_ext is used

The RX Radiotap header length was not calculated properly when reporting
legacy rates using tx_status_ext.

Fixes: 3d07ffca ("mac80211: add struct ieee80211_tx_status support to ieee80211_add_tx_radiotap_header")
Signed-off-by: John Crispin <john@phrozen.org>
Link: https://lore.kernel.org/r/20190807075949.32414-1-john@phrozen.org


Signed-off-by: Johannes Berg <johannes.berg@intel.com>