bpftool: Refactor xdp attach/detach type judgment

This commit no logical changed, just increases code readability and
facilitates TCX prog expansion, which will be implemented in the next
patch.

Signed-off-by: Tao Chen <chen.dylane@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Quentin Monnet <qmo@kernel.org>
Link: https://lore.kernel.org/bpf/20240721143353.95980-2-chen.dylane@gmail.com


Signed-off-by: Andrii Nakryiko <andrii@kernel.org>