usb: gadget: fsl_qe_udc: Fix fall-through warning for Clang

Fix the following fallthrough warning (powerpc-randconfig):

drivers/usb/gadget/udc/fsl_qe_udc.c:589:4: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/lkml/60ef0750.I8J+C6KAtb0xVOAa%25lkp@intel.com/Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/usb/gadget/udc/fsl_qe_udc.c

@@ -586,6 +586,7 @@ static int qe_ep_init(struct qe_udc *udc,
 			case USB_SPEED_FULL:
 				if (max <= 1023)
 					break;
+				fallthrough;
 			default:
 				goto en_done;
 			}