media: vidtv: add a poor guy's simulation to preBER stats

A typical digital TV stream has errors that are corrected
by Viterbi. While the error rate after Viterbi is usually
zero, with good signals, there are some chances of getting
random errors before that, which are auto-corrected by
the error code algorithm.

Add a poor guy's implementation that would show some
noise at the pre-BER part of the demod.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>