[media] af9013: cancel_delayed_work_sync before device removal / kfree

af9013_remove  was calling  kfree(state)  with  possibly still  active
schedule_delayed_work(&state->statistics_work).   A   similar  bug  in
si2157 caused kernel panics in call_timer_fn e.g. after rmmod cx23885.

Signed-off-by: Ernst Martin Witte <emw-linux-kernel@nocabal.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/dvb-frontends/af9013.c

@@ -1344,6 +1344,10 @@ err:
 static void af9013_release(struct dvb_frontend *fe)
 {
 	struct af9013_state *state = fe->demodulator_priv;
+
+	/* stop statistics polling */
+	cancel_delayed_work_sync(&state->statistics_work);
+
 	kfree(state);
 }