wifi: radiotap: document ieee80211_get_radiotap_len() return value

Document the return value of ieee80211_get_radiotap_len() in
the proper kernel-doc format.

Link: https://msgid.link/20240515093852.143aadfdb094.I8795ec1e8cfd7106d58325fb514bae92625fb45c@changeidSigned-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/ieee80211_radiotap.h

@@ -582,6 +582,7 @@ enum ieee80211_radiotap_eht_usig_tb {
 /**
  * ieee80211_get_radiotap_len - get radiotap header length
  * @data: pointer to the header
+ * Return: the radiotap header length
  */
 static inline u16 ieee80211_get_radiotap_len(const char *data)
 {