usb: fix dwc3 build when USB_GADGET_DWC3 is not enabled

Fix build error when CONFIG_USB_GADGET_DWC3 is not enabled:

ERROR: "dwc3_send_gadget_ep_cmd" [drivers/usb/dwc3/dwc3.ko] undefined!

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Acked-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>