usb: dwc3: Makefile: Don't build gadget code if USB_GADGET is disabled

It is pointless to build gadget driver if USB_GADGET is disabled.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Acked-by: Marek Vasut <marex@denx.de>

drivers/usb/dwc3/Makefile

@@ -6,7 +6,7 @@ obj-$(CONFIG_USB_DWC3)			+= dwc3.o
 
 dwc3-y					:= core.o
 
-dwc3-y					+= gadget.o ep0.o
+obj-$(CONFIG_USB_DWC3_GADGET)		+= gadget.o ep0.o
 
 obj-$(CONFIG_USB_DWC3_OMAP)		+= dwc3-omap.o
 obj-$(CONFIG_USB_DWC3_PHY_OMAP)		+= ti_usb_phy.o