mac80211: fix compile error when mesh is disabled

mac80211: fix compile error when mesh is disabled

This fixes following compile error seen when
building mac80211 with mesh disabled:

.../backports-5.15.58-1/net/mac80211/agg-rx.c: In function 'ieee80211_send_addba_resp':
...backports-5.15.58-1/net/mac80211/agg-rx.c:255:17: error: 'struct sta_info' has no member named 'mesh'
  255 |         if (!sta->mesh)
      |                 ^~

sta_info.h shows this item as being optional based on flags:

	struct mesh_sta *mesh;

Guard the check to fix this.

Fixes: f96744ba6b ("mac80211: mask nested A-MSDU support for mesh")
Signed-off-by: Koen Vandeputte <koen.vandeputte@citymesh.com>