Fix kernel-doc warnings in FTM due to missing "struct" keyword.

Fixes 109 warnings from <net/cfg80211.h>:
../include/net/cfg80211.h:2838: warning: cannot understand function prototype: 'struct cfg80211_ftm_responder_stats '

and fixes 88 warnings from <net/mac80211.h>:
../include/net/mac80211.h:477: warning: cannot understand function prototype: 'struct ieee80211_ftm_responder_params '

Fixes: 81e54d08 ("cfg80211: support FTM responder configuration/statistics")
Fixes: bc847970 ("mac80211: support FTM responder configuration/statistics")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Pradeep Kumar Chitrapu <pradeepc@codeaurora.org>
Cc: Johannes Berg <johannes.berg@intel.com>
Cc: David Spinadel <david.spinadel@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>