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@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>