usb: gadget: atmel_usba_udc: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: at91_dt_defconfig arm):

drivers/usb/gadget/udc/atmel_usba_udc.c:329:13: warning: this statement may fall through [-Wimplicit-fallthrough=]
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20190805184842.GA8627@embeddedorSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/gadget/udc/atmel_usba_udc.c

@@ -327,6 +327,7 @@ static int usba_config_fifo_table(struct usba_udc *udc)
 	switch (fifo_mode) {
 	default:
 		fifo_mode = 0;
+		/* fall through */
 	case 0:
 		udc->fifo_cfg = NULL;
 		n = 0;