bpf, tcx: Get rid of tcx_link_const

Small clean up to get rid of the extra tcx_link_const() and only retain
the tcx_link().

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20231023185015.21152-1-daniel@iogearbox.net
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>