dmaengine: Save few bytes and increase readability of dma_request_chan()

Split IS_ERR_OR_NULL() check followed by additional conditional
to two simple conditionals. This increases readability and saves memory:

Function                                     old     new   delta
dma_request_chan                             700     697      -3
Total: Before=10224, After=10221, chg -0.03%

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200828144519.14483-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/dma/dmaengine.c

@@ -847,8 +847,10 @@ struct dma_chan *dma_request_chan(struct device *dev, const char *name)
 	}
 	mutex_unlock(&dma_list_mutex);
 
-	if (IS_ERR_OR_NULL(chan))
-		return chan ? chan : ERR_PTR(-EPROBE_DEFER);
+	if (IS_ERR(chan))
+		return chan;
+	if (!chan)
+		return ERR_PTR(-EPROBE_DEFER);
 
 found:
 #ifdef CONFIG_DEBUG_FS