amdgpu/pm: read_sensor() report failure apporpriately

report -ENOTSUPP instead of -EINVAL, so that if userspace
fails to read sensor data can figure it out the failure correctly.

Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Evan Quan <evan.quan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>