dmatest: print message on debug level in case of no error

Let's move the behaviour of printing no error message back to the pre v3.10
times. It means we will use debug level in the described case, and a warning
level otherwise.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dan Williams <djbw@fb.com>