dma: imx-dma: fix signedness bug

mxdmac->channel was unsigned, so check (imxdmac->channel < 0) for
failed imx_dma_request_by_prio() made no sence.  Explicitly check
signed values.
Also, fix uninitialzed use of ret.
Signed-off-by: Vasiliy Kulikov <segooon@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>

drivers/dma/imx-dma.c

@@ -335,8 +335,10 @@ static int __init imxdma_probe(struct platform_device *pdev)
 
 		imxdmac->imxdma_channel = imx_dma_request_by_prio("dmaengine",
 				DMA_PRIO_MEDIUM);
-		if (imxdmac->channel < 0)
+		if ((int)imxdmac->channel < 0) {
+			ret = -ENODEV;
 			goto err_init;
+		}
 
 		imx_dma_setup_handlers(imxdmac->imxdma_channel,
 		       imxdma_irq_handler, imxdma_err_handler, imxdmac);