hwmon: (it87) Clamp negative values to zero in set_fan()

hwmon: (it87) Clamp negative values to zero in set_fan()

set_fan() parses user input with kstrtol() and passes the resulting
value to FAN16_TO_REG() on chips with 16-bit fan support.

Negative fan speeds are not meaningful and should be rejected before
conversion. Worst scenario, one may be able to abuse undefined
behaviour of signed overflow to possibly induce rpm * 2 == 0 in
FAN16_TO_REG(), thus causing a division by zero.

Instead, clamp val < 0 to zero and keep the conversion in its valid
input domain, avoiding unsafe arithmetic in the register conversion
path.

Found by Linux Verification Center (linuxtesting.org) with static
analysis tool SVACE.

Fixes: 17d648bf5786 ("it87: Add support for the IT8716F")
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Link: https://lore.kernel.org/r/20260529141839.1639287-1-n.zhandarovich@fintech.ru
Signed-off-by: Guenter Roeck <linux@roeck-us.net>