bridge: Use PTR_ERR_OR_ZERO instead if(IS_ERR(...)) + PTR_ERR

coccicheck suggested using PTR_ERR_OR_ZERO() and looking at the code.

Fix the following coccicheck warnings:

./net/bridge/br_multicast.c:1295:7-13: WARNING: PTR_ERR_OR_ZERO can be
used.
Reported-by: Abaci <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Zhong <abaci-bugfix@linux.alibaba.com>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Link: https://lore.kernel.org/r/1611542381-91178-1-git-send-email-abaci-bugfix@linux.alibaba.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/bridge/br_multicast.c

@@ -1300,7 +1300,7 @@ static int br_multicast_add_group(struct net_bridge *br,
 	pg = __br_multicast_add_group(br, port, group, src, filter_mode,
 				      igmpv2_mldv1, false);
 	/* NULL is considered valid for host joined groups */
-	err = IS_ERR(pg) ? PTR_ERR(pg) : 0;
+	err = PTR_ERR_OR_ZERO(pg);
 	spin_unlock(&br->multicast_lock);
 
 	return err;