dmaengine: Use negative condition for better readability

When negative condition is in use we may decrease indentation level
and make the main part of logic better visible.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20200226101842.29426-2-andriy.shevchenko@linux.intel.com


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