[media] dvb-usb/technisat-usb2: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.
technisat-usb2 already sync-cancels the only work item it uses and
there's no reason for it to call flush_scheduled_work().  Don't use
it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Patrick Boettcher <pboettcher@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>