selftests/bpf: amend for wrong bpf_wq_set_callback_impl signature

See the previous patch: the API was wrong, we were provided the pointer
to the value, not the actual struct bpf_wq *.

Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>
Link: https://lore.kernel.org/r/20240708-fix-wq-v2-2-667e5c9fbd99@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>