mac80211: document ieee80211_tx_info.pad

Fixes htmldocs warning:

Warning(mac80211.h:379): No description found for parameter 'pad[2]'
Signed-off-by: NKalle Valo <kalle.valo@nokia.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

include/net/mac80211.h

@@ -323,6 +323,7 @@ struct ieee80211_tx_rate {
  * @flags: transmit info flags, defined above
  * @band: the band to transmit on (use for checking for races)
  * @antenna_sel_tx: antenna to use, 0 for automatic diversity
+ * @pad: padding, ignore
  * @control: union for control data
  * @status: union for status data
  * @driver_data: array of driver_data pointers