usb: dwc3: qcom: fix gadget-only builds

A recent change added a dependency to the USB host stack and broke
gadget-only builds of the driver.

Fixes: 6895ea55 ("usb: dwc3: qcom: Configure wakeup interrupts during suspend")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20220804151001.23612-4-johan+linaro@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/dwc3/dwc3-qcom.c

@@ -310,8 +310,11 @@ static enum usb_device_speed dwc3_qcom_read_usb2_speed(struct dwc3_qcom *qcom)
 	 * currently supports only 1 port per controller. So
 	 * this is sufficient.
 	 */
+#ifdef CONFIG_USB
 	udev = usb_hub_find_child(hcd->self.root_hub, 1);
-
+#else
+	udev = NULL;
+#endif
 	if (!udev)
 		return USB_SPEED_UNKNOWN;