dmaengine: dmatest: Describe members of struct dmatest_info

Kernel documentation validator complains that not all members of
struct dmatest_info are being described. Describe them all.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200424161147.16895-6-andriy.shevchenko@linux.intel.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>