drivers: watchdog: wdt_mcux_wwdt: fix window calculation.

Correct the WWDT window value calculation and add proper
range validation.

The window value was incorrectly set to the minimum timeout value.
Per hardware specification, it should be calculated as:
  windowValue = timeoutValue - min_window_ticks

Also add:
- MAX_TIMEOUT boundary checks
- Validation before value assignment

Signed-off-by: Holt Sun <holt.sun@nxp.com>