drm/amdgpu: return 0 by default in amdgpu_pm_load_smu_firmware

Fixes SI cards running on amdgpu.

Fixes: 19290598 ("drm/amd/amdgpu: add RLC firmware to support raven1 refresh")
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=110883


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