dmaengine: consistently return string literal from switch-case

dmaengine: consistently return string literal from switch-case

There is no need to have 'break;' statement in the default case followed by
return certain string literal when all other cases have returned the string
literals. So, refactor it accordingly.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20200226101842.29426-4-andriy.shevchenko@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>