igb: Fix position of assignment to *ring

Assignment to *ring should be done after correctness check of the
argument queue.

Fixes: 91db3642 ("igb: Refactor igb_configure_cbs()")
Signed-off-by: Jedrzej Jagielski <jedrzej.jagielski@intel.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>