clocksource/drivers/timer-tegra186: Simplify calculating timeleft

clocksource/drivers/timer-tegra186: Simplify calculating timeleft

It is not necessary to use 64-bit operations to calculate the
remaining watchdog timeout. Simplify to use 32-bit operations,
and add comments explaining why there will be no overflow.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Jon Hunter <jonathanh@nvidia.com>
Cc: Pohsun Su <pohsuns@nvidia.com>
Cc: Robert Lin <robelin@nvidia.com>
Link: https://lore.kernel.org/r/20250614175556.922159-2-linux@roeck-us.net