Use drupal_get_form as menu callback

Well first of all the $path variable is used several more times in browserid_login_button().

And about not using drupal_get_form() as the menu callback -- definitely not a bug, doing it this way works -- I tried to use drupal_get_form as the callback in another module I was working on recently and discovered that it now returns the form array instead of the rendered form so it wasn't working as the callback.

If it works, fine. I don't really care. It's 5 lines of code. It doesn't slow anything down -- it's just an administrative menu. But fix the patch and I'll try it.