net/mld: remove duplicate net_unlock in mld_gendog_work

Remove a redundant net_unlock() call in the early return path of
mld_gendog_work(). The network lock is already released at the
common exit path, causing a double-unlock.

Signed-off-by: zhanghongyu <zhanghongyu@xiaomi.com>

net/mld/mld_timer.c

@@ -128,7 +128,6 @@ static void mld_gendog_work(FAR void *arg)
        */
 
       fwarn("WARNING: No device associated with ifindex=%d\n", ifindex);
-      net_unlock();
       return;
     }