edid.py: sample SCL only every 64 clock cycles, to avoid bouncing

Possibly due to SCL rising fairly slowly (in the 0.5-1 us range),
bouncing has been observed while crossing the "forbidden" region
between Vil(max) and Vih(min).

By lowering the sample rate from once per system clock to once
every 64 clock cycles, we make sure we sample at most once during
the bounce interval and thus never see a false edge. (Although we
may see a rising edge one sample time late, which is perfectly
harmless.)