HID: bpf: prevent the same struct_ops to be attached more than once

If the struct_ops is already attached, we should bail out or we will
end up in various locks and pointer issues while unregistering.

Link: https://patch.msgid.link/20240723-fix-6-11-bpf-v1-3-b9d770346784@kernel.org


Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>