watchdog: sp805: Report correct timeleft at maximum

sp805_wdt::load_val is of type unsigned int. When the interrupt is
inactive wdt_timeleft adds one to the value, and then adds that to the
value used to calculate the time remaining. Unfortunately it's not
unlikely that load_val contains LOAD_MAX, which is 0xFFFFFFFF and wraps
to zero when one is added to it, resulting in the time left being
understated by about 21.7s. Fix this by ensuring the addition happens as
64-bit.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20241203095744.3001795-1-mac@mcrowe.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>