[media] fix dvb_frontend_sleep_until() logic

As pointed by Laurent Navet:
	"Calling ktime_add_us() seems useless as is only useful
	 for it's return value which is ignored."

That's reported by coverity CID 1309761.

Laurent proposed to just remove ktime_add_us, but the fact is that
the logic of this function is broken. Instead, we need to use the
value of the timeout, and ensure that it will work on the loops
to emulate the legacy DiSEqC ioctl (FE_DISHNETWORK_SEND_LEGACY_CMD).

Please notice that the logic was also broken if, for any reason,
msleep() would sleep a little less than what it was expected, as
newdelta would be smaller than delta, and udelay() would not be called.

It should also be noticed that nobody noticed that trouble before
likely because the FE_DISHNETWORK_SEND_LEGACY_CMD is not used
anymore by modern DVB applications.

Reported-by: Laurent Navet <laurent.navet@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

drivers/media/dvb-core/dvb_frontend.c

@@ -899,14 +899,13 @@ static void dvb_frontend_stop(struct dvb_frontend *fe)
  */
 void dvb_frontend_sleep_until(ktime_t *waketime, u32 add_usec)
 {
-	s32 delta, newdelta;
+	s32 delta;
 
-	ktime_add_us(*waketime, add_usec);
+	*waketime = ktime_add_us(*waketime, add_usec);
 	delta = ktime_us_delta(ktime_get_real(), *waketime);
 	if (delta > 2500) {
 		msleep((delta - 1500) / 1000);
-		newdelta = ktime_us_delta(ktime_get_real(), *waketime);
-		delta = (newdelta > delta) ? 0 : newdelta;
+		delta = ktime_us_delta(ktime_get_real(), *waketime);
 	}
 	if (delta > 0)
 		udelay(delta);