igb: refactor XDP registration

On changing the RX ring parameters igb uses a hack to avoid a warning
when calling xdp_rxq_info_reg via igb_setup_rx_resources.  It just
clears the struct xdp_rxq_info content.

Instead, change this to unregister if we're already registered.  Align
code to the igc code.

Fixes: 9cbc948b ("igb: add XDP support")
Signed-off-by: Corinna Vinschen <vinschen@redhat.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Tested-by: Sandeep Penigalapati <sandeep.penigalapati@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>