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: 7b0b1a6d

 ("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>