Blackfin: jtag-console: robustify against missing peer

If the other side isn't listening, we should reset the state to ignore
the whole message and not just the part we missed.  This makes it easier
to connect at any time to the jtag console without worrying about the two
sides getting out of sync and thus sending garbage back and forth.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>