hla: if the idcode callback returns 0, treat as a wildcard

Also document the callback accordingly.

Change-Id: I7e8ef481e8b5391b763b7f7187fac023e9fe04df
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1673
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/jtag/drivers/ti_icdi_usb.c

@@ -284,6 +284,7 @@ static int icdi_get_cmd_result(void *handle)
 
 static int icdi_usb_idcode(void *handle, uint32_t *idcode)
 {
+	*idcode = 0;
 	return ERROR_OK;
 }