usb: phy: return -ENXIO when PHY layer isn't enabled

in cases where PHY layer isn't enabled, we want
to still return an error code (actually an error
pointer) so that our users don't need to cope with
either error pointer of NULL.

This will simplify users as below:

-	return IS_ERR(phy) ? PTR_ERR(phy) : -ENODEV;
+	return PTR_ERR(phy);

Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Reported-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Felipe Balbi <balbi@ti.com>

include/linux/usb/phy.h

@@ -197,29 +197,29 @@ extern int usb_bind_phy(const char *dev_name, u8 index,
 #else
 static inline struct usb_phy *usb_get_phy(enum usb_phy_type type)
 {
-	return NULL;
+	return ERR_PTR(-ENXIO);
 }
 
 static inline struct usb_phy *devm_usb_get_phy(struct device *dev,
 	enum usb_phy_type type)
 {
-	return NULL;
+	return ERR_PTR(-ENXIO);
 }
 
 static inline struct usb_phy *usb_get_phy_dev(struct device *dev, u8 index)
 {
-	return NULL;
+	return ERR_PTR(-ENXIO);
 }
 
 static inline struct usb_phy *devm_usb_get_phy_dev(struct device *dev, u8 index)
 {
-	return NULL;
+	return ERR_PTR(-ENXIO);
 }
 
 static inline struct usb_phy *devm_usb_get_phy_by_phandle(struct device *dev,
 	const char *phandle, u8 index)
 {
-	return NULL;
+	return ERR_PTR(-ENXIO);
 }
 
 static inline void usb_put_phy(struct usb_phy *x)