dvb: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.

* Flush the used works directly.

* Replace the deprecated cancel_rearming_delayed_work() +
  flush_scheduled_work() -> cancel_delayed_work_sync().

* Make sure mantis->uart_work isn't running on exit.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Mauro Carvalho Chehab <mchehab@infradead.org>
Cc: linux-media@vger.kernel.org