iavf: remove an unneeded variable

The variable `ret_code' used for returning is never changed in function
`iavf_shutdown_adminq'. So that it can be removed and just return its
initial value 0 at the end of `iavf_shutdown_adminq' function.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>