spi/mpc8xxx: don't check platform_get_irq's return value against zero

platform_get_irq returns -ENXIO on failure, so !irq was probably
always true.  Make irq a signed variable and compare irq <= 0.  Note
that a return value of zero is still handled as error even though this
could mean irq0.

This is a followup to 305b3228 that
changed the return value of platform_get_irq from 0 to -ENXIO on error.
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/spi_mpc8xxx.c

@@ -1328,7 +1328,7 @@ static struct of_platform_driver of_mpc8xxx_spi_driver = {
 static int __devinit plat_mpc8xxx_spi_probe(struct platform_device *pdev)
 {
 	struct resource *mem;
-	unsigned int irq;
+	int irq;
 	struct spi_master *master;
 
 	if (!pdev->dev.platform_data)
@@ -1339,7 +1339,7 @@ static int __devinit plat_mpc8xxx_spi_probe(struct platform_device *pdev)
 		return -EINVAL;
 
 	irq = platform_get_irq(pdev, 0);
-	if (!irq)
+	if (irq <= 0)
 		return -EINVAL;
 
 	master = mpc8xxx_spi_probe(&pdev->dev, mem, irq);