hwmon: (asus-ec-sensors) check sensor index in read_string()

hwmon: (asus-ec-sensors) check sensor index in read_string()

Prevent a potential invalid memory access when the requested sensor
is not found.

find_ec_sensor_index() may return a negative value (e.g. -ENOENT),
but its result was used without checking, which could lead to
undefined behavior when passed to get_sensor_info().

Add a proper check to return -EINVAL if sensor_index is negative.

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

Fixes: d0ddfd241e57 ("hwmon: (asus-ec-sensors) add driver for ASUS EC")
Signed-off-by: Alexei Safin <a.safin@rosa.ru>
Link: https://lore.kernel.org/r/20250424202654.5902-1-a.safin@rosa.ru
[groeck: Return error code returned from find_ec_sensor_index]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>