drm/amd/pm: fix error flow in sensor fetching

Sensor fetching functions should return an signed int to
handle errors properly.

Reviewed-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reported-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>