watchdog: airoha: Prevent division by zero when clock frequency is zero

In the Linux kernel, the following vulnerability has been resolved: watchdog: airoha: Prevent division by zero when clock frequency is zero clk_get_rate() can return 0 when the clock provider is not properly configured or the clock is unmanaged. The driver uses wdt_freq as a divisor directly in airoha_wdt_probe() to compute max_timeout and in airoha_wdt_get_timeleft() to compute the remaining time, which results in a division by zero. Add a check for wdt_freq == 0 in probe and return -EINVAL with dev_err_probe() to prevent the division by zero and provide a diagnostic message.