selftests/bpf/test_offload.py: Fix expected case of extack messages

Commit 7f0a8382 ("bpf, xdp: Maintain info on attached XDP BPF programs
in net_device") changed the case of some of the extack messages being
returned when attaching of XDP programs failed. This broke test_offload.py,
so let's fix the test to reflect this.

Fixes: 7f0a8382 ("bpf, xdp: Maintain info on attached XDP BPF programs in net_device")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/bpf/160752226175.110217.11214100824416344952.stgit@toke.dk