drm/amdgpu: correct ta header v2 ucode init start address

resolve bug calculating fw start address within binary

Reviewed-by: Guchun Chen <guchun.chen@amd.com>
Signed-off-by: John Clements <john.clements@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>