Instead of using locking per-channel, just lock the entire ISDN net work
interface as an entity, getting rid of weird locking.
  
Basically, ::hard_start_xmit() is already serialized by the network core,
so we could just rely on that. However, we want to send keep alive
frames, PPP messages from ipppd and such things directly without going
through the network stack, so we additionally lock
isdn_net_local->xmit_lock in hard_start_xmit(), which is taken in
the other paths dealing with transmitting frames as well.