[media] cxd2841er: don't expose a dvbv5 stats to userspace if not available

The current code will expose a zero value if one of the stats is
not available, but this is not what userspace expects. Instead,
if something goes wrong on providing some stats, it should be
changing the scale to FE_SCALE_NOT_AVAILABLE.

So, change the logic to do the right thing.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>