cochran: do a partial header de-scramble

This descrambles at least parts of the header data.  Some of it has the
same pattern of data 4kB apart, it may be that there is a dive hiding in
there too (ie what I currently call a "header" may in fact be a header
_plus_ a dive).

But the 4kB thing may well be an artifact of the crazy scrambling code
itself.  Who knows what kind of chunking the Cochran Analyst
"encryption" uses.

As with the dive data, there seems to be some offset differences between
different CAN files.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>