spi/dw_spi: don't treat NULL clk as an error

clk_get() returns a struct clk cookie to the driver and some platforms
may return NULL if they only support a single clock.  clk_get() has only
failed if it returns a ERR_PTR() encoded pointer.
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/dw_spi_mmio.c

@@ -9,6 +9,7 @@
  */
 
 #include <linux/clk.h>
+#include <linux/err.h>
 #include <linux/interrupt.h>
 #include <linux/platform_device.h>
 #include <linux/slab.h>
@@ -68,8 +69,8 @@ static int __devinit dw_spi_mmio_probe(struct platform_device *pdev)
 	}
 
 	dwsmmio->clk = clk_get(&pdev->dev, NULL);
-	if (!dwsmmio->clk) {
-		ret = -ENODEV;
+	if (IS_ERR(dwsmmio->clk)) {
+		ret = PTR_ERR(dwsmmio->clk);
 		goto err_irq;
 	}
 	clk_enable(dwsmmio->clk);