ARM: shmobile: marzen: Use error values in usb_power_*

This patch updates the marzen board code as if USB PHY isn't enabled
they phy will have a value set by ERR_PTR() rather than be NULL.

Without this patch a NULL pointer dereference  and kernel panic
occurs on initialisation of USB on marzen.

This resolves a regression introduced in 3.10-rc1 by
b7fa5c2a
("usb: phy: return -ENXIO when PHY layer isn't enabled").
Tested-by: Nguyen Hong Ky <nh-ky@jinso.co.jp>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>

arch/arm/mach-shmobile/board-marzen.c

@@ -212,8 +212,8 @@ static struct platform_device *marzen_devices[] __initdata = {
 static struct usb_phy *phy;
 static int usb_power_on(struct platform_device *pdev)
 {
-	if (!phy)
-		return -EIO;
+	if (IS_ERR(phy))
+		return PTR_ERR(phy);
 
 	pm_runtime_enable(&pdev->dev);
 	pm_runtime_get_sync(&pdev->dev);
@@ -225,7 +225,7 @@ static int usb_power_on(struct platform_device *pdev)
 
 static void usb_power_off(struct platform_device *pdev)
 {
-	if (!phy)
+	if (IS_ERR(phy))
 		return;
 
 	usb_phy_shutdown(phy);