powerpc/512x: Use dma_request_chan() instead dma_request_slave_channel()

dma_request_slave_channel() is a wrapper on top of dma_request_chan()
eating up the error code.

By using dma_request_chan() directly the driver can support deferred
probing against DMA.

Signed-off-by: Peter Ujfalusi <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/powerpc/platforms/512x/mpc512x_lpbfifo.c

···
 	memset(&lpbfifo, 0, sizeof(struct lpbfifo_data));
 	spin_lock_init(&lpbfifo.lock);
 
-	lpbfifo.chan = dma_request_slave_channel(&pdev->dev, "rx-tx");
-	if (lpbfifo.chan == NULL)
-		return -EPROBE_DEFER;
+	lpbfifo.chan = dma_request_chan(&pdev->dev, "rx-tx");
+	if (IS_ERR(lpbfifo.chan))
+		return PTR_ERR(lpbfifo.chan);
 
 	if (of_address_to_resource(pdev->dev.of_node, 0, &r) != 0) {
 		dev_err(&pdev->dev, "bad 'reg' in 'sclpc' device tree node\n");