Most other modules in the recieve path are reset when signal_status goes
low. This one didn't, and it was difficult to test because of this. In
particular, we need to ensure that this module behaves correctly when
switching between different bitstreams (such as for loopback).
While implementing this, I found some bugs in the way that nrzi_valid
was handled: it wasn't saved from when the transfer actually happened,
and the data wasn't qualified properly.
Signed-off-by: Sean Anderson <seanga2@gmail.com>