The radeon_asic_ring structures are never modified, so declare them as
const.

Done with the help of Coccinelle.
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NJulia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>