spi: tegra: checking for ERR_PTR instead of NULL

dma_request_slave_channel() returns NULL on error and not ERR_PTRs.
I've fixed this by using dma_request_slave_channel_reason() which does
return ERR_PTRs.

Fixes: a915d150f68d ('spi: tegra: convert to standard DMA DT bindings')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
diff --git a/drivers/spi/spi-tegra20-slink.c b/drivers/spi/spi-tegra20-slink.c
index dd6f26c..a728bb8 100644
--- a/drivers/spi/spi-tegra20-slink.c
+++ b/drivers/spi/spi-tegra20-slink.c
@@ -630,8 +630,8 @@
 	int ret;
 	struct dma_slave_config dma_sconfig;
 
-	dma_chan = dma_request_slave_channel(tspi->dev,
-					     dma_to_memory ? "rx" : "tx");
+	dma_chan = dma_request_slave_channel_reason(tspi->dev,
+						dma_to_memory ? "rx" : "tx");
 	if (IS_ERR(dma_chan)) {
 		ret = PTR_ERR(dma_chan);
 		if (ret != -EPROBE_DEFER)