drm/amdgpu: remove set but not used variable 'ring'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/gpu/drm/amd/amdgpu/psp_v10_0.c: In function 'psp_v10_0_ring_stop':
drivers/gpu/drm/amd/amdgpu/psp_v10_0.c:230:19: warning:
 variable 'ring' set but not used [-Wunused-but-set-variable]

drivers/gpu/drm/amd/amdgpu/psp_v3_1.c: In function 'psp_v3_1_ring_stop':
drivers/gpu/drm/amd/amdgpu/psp_v3_1.c:359:19: warning:
 variable ‘ring’ set but not used [-Wunused-but-set-variable]

It not used since commit
4ef72453 ("drm/amdgpu: added api for stopping psp ring (v2)")

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>