i40e: Make i40e_shutdown_adminq() return void

Fix the following coccicheck warning:

drivers/net/ethernet/intel/i40e/i40e_adminq.c:699:13-21: Unneeded
variable: "ret_code". Return "0" on line 710

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>