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: a915d150 ('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>

drivers/spi/spi-tegra20-slink.c

@@ -630,7 +630,7 @@ static int tegra_slink_init_dma_param(struct tegra_slink_data *tspi,
 	int ret;
 	struct dma_slave_config dma_sconfig;
 
-	dma_chan = dma_request_slave_channel(tspi->dev,
+	dma_chan = dma_request_slave_channel_reason(tspi->dev,
 						dma_to_memory ? "rx" : "tx");
 	if (IS_ERR(dma_chan)) {
 		ret = PTR_ERR(dma_chan);