drm/amdgpu: remove set but not used variable 'ring' in psp_v11_0_ring_stop

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

drivers/gpu/drm/amd/amdgpu/psp_v11_0.c: In function 'psp_v11_0_ring_stop':
drivers/gpu/drm/amd/amdgpu/psp_v11_0.c:309:19: warning:
 variable 'ring' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>