usb: of: fix build breakage on !OF

If OF is disabled, we will try to define a stub for
of_usb_get_dr_mode_by_phy(), however that missed a
static inline annotation which made us redefine the
stub over and over again. Fix that.

Fixes: 98bfb394 ("usb: of: add an api to get
	dr_mode by the phy node")
Signed-off-by: Felipe Balbi <balbi@ti.com>

include/linux/usb/of.h

@@ -17,7 +17,8 @@ bool of_usb_host_tpl_support(struct device_node *np);
 int of_usb_update_otg_caps(struct device_node *np,
 			struct usb_otg_caps *otg_caps);
 #else
-enum usb_dr_mode of_usb_get_dr_mode_by_phy(struct device_node *phy_np)
+static inline enum usb_dr_mode
+of_usb_get_dr_mode_by_phy(struct device_node *phy_np)
 {
 	return USB_DR_MODE_UNKNOWN;
 }