selftests/bpf: Fix typo in subtest_basic_usdt after merge conflict

Use proper 'called' variable name.

Fixes: ae28ed45 ("Merge tag 'bpf-next-6.18' of git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next"

)
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/aN0JVRynHxqKy4lw@krava/


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