i40evf: remove unnecessary error checking against i40e_shutdown_adminq

The i40e_shutdown_adminq function never returns failure. There is no need to
check the non-0 return value. Clean up the unnecessary error checking and
warning against it.

Change-ID: Ibb616f09cfb93bd1a872ebf3241a15fb8354b31b
Signed-off-by: Lihong Yang <lihong.yang@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>