Add description for struct member 'agg' to prevent a kernel-doc
warning.

mac80211.h:2289: warning: Function parameter or member 'agg' not described in 'ieee80211_link_sta'

Fixes: 4c51541d ("wifi: mac80211: keep A-MSDU data in sta and per-link")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Johannes Berg <johannes@sipsolutions.net>
Cc: Benjamin Berg <benjamin.berg@intel.com>
Cc: linux-wireless@vger.kernel.org
Link: https://lore.kernel.org/r/20230710230312.31197-10-rdunlap@infradead.org
[reword the kernel-doc comment]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>