mISDN: Fix wrong usage of flush_work_sync while holding locks

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 <[email protected]>
Cc: <[email protected]>
Signed-off-by: David S. Miller <[email protected]>