mISDN: use DECLARE_COMPLETION_ONSTACK for non-constant completion

The _ONSTACK variant should be used for on-stack completion,
otherwise it will break lockdep.
 
Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Acked-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>