cfg80211: fix a few kernel-doc warnings

Fix multiple kernel-doc warnings in cfg80211.h.

cfg80211.h:363: warning: missing initial short description on line:
 * struct ieee80211_sband_iftype_data
cfg80211.h:6743: warning: missing initial short description on line:
 * cfg80211_vendor_cmd_get_sender

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20210417060142.1648-1-rdunlap@infradead.org


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