There isn't a way of doing this - other than installing an older version of Files.
This behaviour (i.e. only one search mode) was introduce at r2445 following bug report https://bugs.launchpad.net/pantheon-files/+bug/1604300.
However, only the first part of the intended changes have been released; the second part is still awaiting review. The second part will fix the sorting of the search results such that the first result, selected by default, will be the same as what you would have got with the old search by typing (current folder only, starting with search term).
Hopefully this will remove any need to be able to disable the new behaviour (it is unlikely any such new options/settings would be accepted into elementaryos).