drm/amd/amdgpu: amdgpu_hwmon_show_temp: initialize temp

Reported by smatch:
drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c:1496 amdgpu_hwmon_show_temp() error: uninitialized symbol 'temp'.

Signed-off-by: Ernst Sjöstrand <ernstp@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>