net: marvell: prestera: Fix a NULL vs IS_ERR() check in some functions

rhashtable_lookup_fast() returns NULL when failed instead of error
pointer.

Fixes: 396b80cb ("net: marvell: prestera: Add neighbour cache accounting")
Fixes: 0a23ae23 ("net: marvell: prestera: Add router nexthops ABI")
Signed-off-by: Shang XiaoJing <shangxiaojing@huawei.com>
Link: https://lore.kernel.org/r/20221125012751.23249-1-shangxiaojing@huawei.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

drivers/net/ethernet/marvell/prestera/prestera_router.c

@@ -457,7 +457,7 @@ prestera_kern_neigh_cache_find(struct prestera_switch *sw,
 	n_cache =
 	 rhashtable_lookup_fast(&sw->router->kern_neigh_cache_ht, key,
 				__prestera_kern_neigh_cache_ht_params);
-	return IS_ERR(n_cache) ? NULL : n_cache;
+	return n_cache;
 }
 
 static void

drivers/net/ethernet/marvell/prestera/prestera_router_hw.c

@@ -330,7 +330,7 @@ prestera_nh_neigh_find(struct prestera_switch *sw,
 
 	nh_neigh = rhashtable_lookup_fast(&sw->router->nh_neigh_ht,
 					  key, __prestera_nh_neigh_ht_params);
-	return IS_ERR(nh_neigh) ? NULL : nh_neigh;
+	return nh_neigh;
 }
 
 struct prestera_nh_neigh *
@@ -484,7 +484,7 @@ __prestera_nexthop_group_find(struct prestera_switch *sw,
 
 	nh_grp = rhashtable_lookup_fast(&sw->router->nexthop_group_ht,
 					key, __prestera_nexthop_group_ht_params);
-	return IS_ERR(nh_grp) ? NULL : nh_grp;
+	return nh_grp;
 }
 
 static struct prestera_nexthop_group *