mac80211: add missing length field increment when generating Radiotap header

The code generating the Tx Radiotap header when using tx_status_ext was
missing a field increment after setting the VHT bandwidth.

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/20190807075949.32414-4-john@phrozen.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>