[media] dvb-core: preserve the delivery system at cache clear

The changeset 240ab508 is incomplete, as the first thing that
happens at cache clear is to do a memset with 0 to the cache.

So, the delivery system needs to be explicitly preserved there.
Signed-off-by: NMauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/dvb/dvb-core/dvb_frontend.c

@@ -904,8 +904,11 @@ static int dvb_frontend_clear_cache(struct dvb_frontend *fe)
 {
 	struct dtv_frontend_properties *c = &fe->dtv_property_cache;
 	int i;
+	u32 delsys;
 
+	delsys = c->delivery_system;
 	memset(c, 0, sizeof(struct dtv_frontend_properties));
+	c->delivery_system = delsys;
 
 	c->state = DTV_CLEAR;