drm/amdgpu/sriov: fix Tonga load driver failed

Tonga sriov need to use smu to load firmware.
Remove sriov flag because the default return value is zero.

Signed-off-by: Jack Zhang <Jack.Zhang1@amd.com>
Reviewed-by: Trigger Huang <Trigger.Huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>