drm/amdkfd: Reconcile the definition and use of oem_id in struct kfd_topology_device

Currently oem_id is defined as uint8_t[6] and casted to uint64_t*
in some use case. This would lead code scanner to complain about
access beyond. Re-define it in union to enforce 8-byte size and
alignment to avoid potential issue.

Signed-off-by: Michael Chen <michael.chen@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>