igc: Fix TX drops in XDP ZC

Fixes TX frame drops in AF_XDP zero copy mode when budget < 4.
xsk_tx_peek_desc() consumed TX frame and it was ignored because of
low budget. Not even AF_XDP completion was done for dropped frames.

It can be reproduced on i226 by sending 100000x 60 B frames with
launch time set to minimal IPG (672 ns between starts of frames)
on 1Gbit/s. Always 1026 frames are not sent and are missing a
completion.

Fixes: 9acf59a7 ("igc: Enable TX via AF_XDP zero-copy")
Signed-off-by: Zdenek Bouska <zdenek.bouska@siemens.com>
Reviewed-by: Song Yoong Siang <yoong.siang.song@intel.com>
Reviewed-by: Florian Bezdeka <florian.bezdeka@siemens.com>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>