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

In the Linux kernel, the following vulnerability has been resolved: 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. [groeck: Return error code returned from find_ec_sensor_index]