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]