mISDN: L2 timeouts need to be queued as L2 event

To be full preemptiv safe, we cannot handle a L2 timeout in the timer
context itself, we should do all actions via the D-channel thread.

Signed-off-by: Karsten Keil <kkeil@linux-pingi.de>
Signed-off-by: David S. Miller <davem@davemloft.net>