appletalk: fix a coccinella warning in net/appletalk/ddp.c

This warning is introduced by commit 7b30600c ("appletalk:
fix checkpatch error with indent"), So fix it.

Signed-off-by: Wang Weidong <wangweidong1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>