usb: dwc2: allow dwc2 to get built when USB_GADGET=m

This patch allows the gadget portion of the DWC2 driver to get built when
(!USB && USB_GADGET) condition is encountered.
Acked-by: NPaul Zimmerman <paulz@synopsys.com>
Signed-off-by: NDinh Nguyen <dinguyen@opensource.altera.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

drivers/usb/dwc2/Kconfig

 config USB_DWC2
 	bool "DesignWare USB2 DRD Core Support"
-	depends on USB
+	depends on USB || USB_GADGET
 	help
 	  Say Y here if your system has a Dual Role Hi-Speed USB
 	  controller based on the DesignWare HSOTG IP Core.