wifi: mac80211: describe return values in kernel-doc

Add descriptions for two return values for two functions
that are missing them.

Signed-off-by: Johannes Berg <[e-mail]>
Signed-off-by: Gregory Greenman <[e-mail]>
Link: https://lore.kernel.org/r/20230920211508.79307c341723.Ibae386f0354f2e215d4955752ac378acc2466b51@changeid
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/tx.c

···
  * @dev: incoming interface
  *
  * On failure skb will be freed.
+ *
+ * Returns: the netdev TX status (but really only %NETDEV_TX_OK)
  */
 netdev_tx_t ieee80211_subif_start_xmit(struct sk_buff *skb,
 				       struct net_device *dev)

net/mac80211/util.c

···
  * This function calculates the RX timestamp at the given MPDU offset, taking
  * into account what the RX timestamp was. An offset of 0 will just normalize
  * the timestamp to TSF at beginning of MPDU reception.
+ *
+ * Returns: the calculated timestamp
  */
 u64 ieee80211_calculate_rx_timestamp(struct ieee80211_local *local,
 				     struct ieee80211_rx_status *status,