watchdog: tangier: Replace unused constant with a comment

The default timeout value had been left in order to leave some traces
about default setup of watchdog done by firmware.

For better understanding and compiler burden, replace it with a comment.

Suggested-by: Stefan Roese <sr@denx.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>