(powerpc/512) Fix possible `dma_unmap_single()` on uninitialized pointer

If the device configuration fails (if `dma_dev->device_config()`),
`sg_dma_address(&sg)` is not initialized and the jump to `err_dma_prep`
leads to calling `dma_unmap_single()` on `sg_dma_address(&sg)`.

Signed-off-by: Thomas Fourier <[e-mail]>
Reviewed-by: Christophe Leroy <[e-mail]>
Signed-off-by: Madhavan Srinivasan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

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

···
 	dma_conf.src_addr_width = DMA_SLAVE_BUSWIDTH_4_BYTES;
 
 	/* Make DMA channel work with LPB FIFO data register */
-	if (dma_dev->device_config(lpbfifo.chan, &dma_conf)) {
-		ret = -EINVAL;
-		goto err_dma_prep;
-	}
+	if (dma_dev->device_config(lpbfifo.chan, &dma_conf))
+		return -EINVAL;
 
 	sg_init_table(&sg, 1);