sh: fix DMA driver's descriptor chaining and cookie assignment

The SH DMA driver wrongly assigns negative cookies to transfer descriptors,
also, its chaining of partial descriptors is broken. The latter problem is
usually invisible, because maximum transfer size per chunk is 16M, but if you
artificially set this limit lower, the driver fails. Since cookies are also
used in chunk management, both these problems are fixed in one patch. As side
effects a possible memory leak, when descriptors are prepared, but not
submitted, and multiple races have also been fixed.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Acked-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>