mISDN: Fix wrong usage of flush_work_sync while holding locks

It is a bad idea to hold a spinlock and call flush_work_sync.
Move the workqueue cleanup outside the spinlock and use cancel_work_sync,
on closing the channel this seems to be the more correct function.
Remove the never used and constant return value of mISDN_freebchannel.

Signed-off-by: Karsten Keil <keil@b1-systems.de>
Cc: <stable@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>