selftests/bpf: Fix bpf_prog_detach2 usage in test_lirc_mode2

Commit e9fc3ce9 ("libbpf: Streamline error reporting for high-level
APIs") redefined the way that bpf_prog_detach2() returns. Therefore, adapt
the usage in test_lirc_mode2_user.c.

Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250828-selftests-bpf-v1-1-c7811cd8b98c@suse.com