usb: fix build error without CONFIG_USB_PHY

on i386:

drivers/built-in.o: In function `ci_hdrc_probe':
core.c:(.text+0x20446b): undefined reference to `of_usb_get_phy_mode'

Signed-off-by: Peter Chen <peter.chen@freescale.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/usb/of.h

@@ -10,19 +10,23 @@
 #include <linux/usb/otg.h>
 #include <linux/usb/phy.h>
 
-#ifdef CONFIG_OF
-enum usb_phy_interface of_usb_get_phy_mode(struct device_node *np);
+#if IS_ENABLED(CONFIG_OF)
 enum usb_dr_mode of_usb_get_dr_mode(struct device_node *np);
 #else
-static inline enum usb_phy_interface of_usb_get_phy_mode(struct device_node *np)
-{
-	return USBPHY_INTERFACE_MODE_UNKNOWN;
-}
-
 static inline enum usb_dr_mode of_usb_get_dr_mode(struct device_node *np)
 {
 	return USB_DR_MODE_UNKNOWN;
 }
 #endif
 
+#if IS_ENABLED(CONFIG_OF) && IS_ENABLED(CONFIG_USB_PHY)
+enum usb_phy_interface of_usb_get_phy_mode(struct device_node *np);
+#else
+static inline enum usb_phy_interface of_usb_get_phy_mode(struct device_node *np)
+{
+	return USBPHY_INTERFACE_MODE_UNKNOWN;
+}
+
+#endif
+
 #endif /* __LINUX_USB_OF_H */