igc: Fix XSK queue NAPI ID mapping

In commit b6596985 ("igc: Link queues to NAPI instances"), the XSK
queues were incorrectly unmapped from their NAPI instances. After
discussion on the mailing list and the introduction of a test to codify
the expected behavior, we can see that the unmapping causes the
check_xsk test to fail:

NETIF=enp86s0 ./tools/testing/selftests/drivers/net/queues.py

[...]
  # Check|     ksft_eq(q.get('xsk', None), {},
  # Check failed None != {} xsk attr on queue we configured
  not ok 4 queues.check_xsk

After this commit, the test passes:

  ok 4 queues.check_xsk

Note that the test itself is only in net-next, so I tested this change
by applying it to my local net-next tree, booting, and running the test.

Cc: stable@vger.kernel.org
Fixes: b6596985 ("igc: Link queues to NAPI instances")
Signed-off-by: Joe Damato <jdamato@fastly.com>
Reviewed-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Tested-by: Mor Bar-Gabay <morx.bar.gabay@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>