dmaengine: edma: Do not change the error code returned from edma_alloc_slot

In case of edma_alloc_slot() failure during probe we should return the error
unchanged to make debugging easier.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>