i40e: Fix use-after-free in i40e_client_subtask()

Currently the call to i40e_client_del_instance frees the object
pf->cinst, however pf->cinst->lan_info is being accessed after
the free. Fix this by adding the missing return.

Addresses-Coverity: ("Read from pointer after free")
Fixes: 7b0b1a6 ("i40e: Disable iWARP VSI PETCP_ENA flag on netdev down events")
Signed-off-by: Yunjian Wang <wangyunjian@huawei.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>

drivers/net/ethernet/intel/i40e/i40e_client.c

@@ -375,6 +375,7 @@ void i40e_client_subtask(struct i40e_pf *pf)
 				clear_bit(__I40E_CLIENT_INSTANCE_OPENED,
 					  &cdev->state);
 				i40e_client_del_instance(pf);
+				return;
 			}
 		}
 	}