igb: XDP xmit back fix error code

The igb XDP xmit back function should only return
defined error codes.

Fixes: 9cbc948b ("igb: add XDP support")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Sven Auhagen <sven.auhagen@voleatech.de>
Tested-by: Sandeep Penigalapati <sandeep.penigalapati@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>