usb: zynqmp: Fix build warnings

The driver does "return 0" in function with void type.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Siva Durga Prasad Paladugu <sivadur@xilinx.com>

drivers/usb/host/xhci-zynqmp.c

@@ -139,5 +139,5 @@ void xhci_hcd_stop(int index)
 	 * sw. But this support may be added in future socs.
 	 */
 
-	return 0;
+	return;
 }