dmaengine: dmatest: Print error codes as signed value

dmaengine: dmatest: Print error codes as signed value

When we got an error from DMA mapping API we convert a negative value
to unsigned long type and hence make user confused:

  result #1: 'src mapping error' with src_off=0x19a72 dst_off=0xea len=0xccf4 (18446744073709551604)

Change this to print error codes as signed values.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200907101306.61824-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>