Fix this warning by adding 'ring' arguments to kdoc.

gcc with W=1
drivers/gpu/drm/amd/amdgpu/sdma_v6_0.c:1128: warning: Function parameter or member 'ring' not described in 'sdma_v6_0_ring_pad_ib'

Cc: Felix Kuehling <Felix.Kuehling@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>