mISDN: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.
Directly flush ch->workq when freeing channel and cancel it on
release.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: netdev@vger.kernel.org