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

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