drm/amdgpu: remove dead code

The less than zero comparison of unsigned variable "value" is never
true. Remove dead code.

Fixes: c3ed0e72 ("drm/amdgpu: added a sysfs interface for thermal throttling")
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>