How do I download a specific git commit from a repository?

This would appear to be impossible. According to a discussion on kernel.org, the protocol will only allow named refs to be fetched. If you don't wish to download the snapshot from the git website, you'll have to clone the entire repo.

(You may wish to read the manuals for git-fetch and git-ls-remote.)

In the general case, you can do this using the --remote flag to git archive, like so:

$ git archive -o repo.tar --remote=<repo url> <commit id>

So in your example, you'd use:

$ git archive -o repo.tar --remote=git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-2.6.git ee9c5cfad29c8a13199962614b9b16f1c4137ac9

That'll give you the state of the repo at that point in time. Note that you won't get the whole repo, so you can't actually interact with the upstream repo with what you've downloaded.

However, using git archive remotely has to be enabled server-side, and it isn't on the Linux kernel's Git server. You can, however, grab a copy by using a URL of the form http://git.kernel.org/?p=<path to repo>;a=snapshot;h=<commit id>;sf=tgz. So for your repo, you could use, say, wget or curl to grab the file using that URL.