drm/amdgpu/powerplay/tonga: return an error if copying to smc fails

Rather than just silently dropping it.  Also fixes a set but
unused variable warning.
Reviewed-by: NEvan Quan <evan.quan@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/pm/powerplay/smumgr/tonga_smumgr.c

@@ -2545,7 +2545,7 @@ static int tonga_thermal_setup_fan_table(struct pp_hwmgr *hwmgr)
 					(uint32_t)sizeof(fan_table),
 					SMC_RAM_END);
 
-	return 0;
+	return res;
 }