usb: musb/ux500: 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.
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Bin Liu <b-liu@ti.com>
Link: https://lore.kernel.org/r/20200115132547.364-19-b-liu@ti.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/musb/ux500_dma.c

@@ -310,9 +310,9 @@ static int ux500_dma_controller_start(struct ux500_dma_controller *controller)
 			dma_channel->max_len = SZ_16M;
 
 			ux500_channel->dma_chan =
-				dma_request_slave_channel(dev, chan_names[ch_num]);
+				dma_request_chan(dev, chan_names[ch_num]);
 
-			if (!ux500_channel->dma_chan)
+			if (IS_ERR(ux500_channel->dma_chan))
 				ux500_channel->dma_chan =
 					dma_request_channel(mask,
 							    data ?