tipc: fix return value check in tipc_mcast_send_sync()

Fix the return value check which testing the wrong variable
in tipc_mcast_send_sync().

Fixes: c55c8eda ("tipc: smooth change between replicast and broadcast")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>