dmaengine: edma: Fix paRAM slot allocation for entry channel 0

edma_alloc_slot was not checking the channel mapping support existence when
slot 0 has been requested (used as entry slot for channel/event 0).

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