cfg80211: Add documentation for antenna ops

The last patch with the same title was for mac80211 ops, accidentally.
Signed-off-by: NBruno Randolf <br1@einfach.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>

include/net/cfg80211.h

@@ -1154,6 +1154,13 @@ struct cfg80211_pmksa {
  * @mgmt_frame_register: Notify driver that a management frame type was
  *	registered. Note that this callback may not sleep, and cannot run
  *	concurrently with itself.
+ *
+ * @set_antenna: Set antenna configuration (tx_ant, rx_ant) on the device.
+ *	Parameters are bitmaps of allowed antennas to use for TX/RX. Drivers may
+ *	reject TX/RX mask combinations they cannot support by returning -EINVAL
+ *	(also see nl80211.h @NL80211_ATTR_WIPHY_ANTENNA_TX).
+ *
+ * @get_antenna: Get current antenna configuration from device (tx_ant, rx_ant).
  */
 struct cfg80211_ops {
 	int	(*suspend)(struct wiphy *wiphy);