drm/amd/amdgpu: Fix amdgpu_set_pp_od_clk_voltage error check

Reported by smatch:
drivers/gpu/drm/amd/amdgpu/amdgpu_pm.c:693 amdgpu_set_pp_od_clk_voltage() error: uninitialized symbol 'ret'.

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