bpf/selftests: Fix ASSERT_OK condition check in uprobe_syscall test

Fixing ASSERT_OK condition check in uprobe_syscall test,
otherwise we return from test on pipe success.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/20240726180847.684584-1-jolsa@kernel.org