[media] dib8000: be sure that stats are available before reading them

On dib8000, the BER statistics are updated on every 1.25e6 bits.
Adjust the code to only update the statistics after having it
done.

Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>
Acked-by: Patrick Boettcher <pboettcher@kernellabs.com>