dmaengine: edma: Rename bitfields for slot and channel usage tracking

The names chosen for the bitfields were quite confusing and given no real
information on what they are used for...

edma_inuse -> slot_inuse: tracks the slot usage/availability
edma_unused -> channel_unused: tracks the channel usage/availability

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