dmaengine: Fix memory leak in dma_async_device_register

The atomic reference counter, idr_ref, is leaked when there are no
channels available for a DMA device. Free idr_ref in this case to fix
the memory leak.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>