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>

drivers/gpu/drm/amd/amdgpu/psp_v11_0.c

@@ -306,11 +306,8 @@ static int psp_v11_0_ring_stop(struct psp_context *psp,
 			      enum psp_ring_type ring_type)
 {
 	int ret = 0;
-	struct psp_ring *ring;
 	struct amdgpu_device *adev = psp->adev;
 
-	ring = &psp->km_ring;
-
 	/* Write the ring destroy command to C2PMSG_64 */
 	WREG32_SOC15(MP0, 0, mmMP0_SMN_C2PMSG_64, GFX_CTRL_CMD_ID_DESTROY_RINGS);