This does not adress the actual question, but might be useful, if somebody wants to trace back the literature as aluded to in another answer.
Peter Roquette has writteen a series of articles entitled
The Riemann hypothesis in characteristic p, its origin and development
several several parts, total length close to 200 pages.
Bibliographic Bibliographic details and pdf are avalaible here: http://www.rzuser.uni-heidelberg.de/~ci3/manu.html (see papers 26,27,36).
I am not familiar with the content of these paper, the following is based on a very rough browsing.
The covered time span seems to be roughly 1920-40.
E. Artin's (1921) and F.K. Schmidt's (1925) theses seem the most important starting points. However, several other names are mentioned as well; H. Hasse worked on the Riemann Hyothesis (in this context) around 33/34; referring (freely translated) to Artin and Schmidt zeta-functions.
F.K. Schmidt: Zur Zahlentheorie in Koorpern der Charakteristik p. (Vorlaeufige Mitteilung.) Sitz.-Ber. phys. med. Soz. Erlangen 58/59 (1926/27) 159–172
is mentioned as the paper introducing the zeta-function for function fields (fin.gen. and trans. deg. 1).
As motivation for these investigations, the analogy with the number field case is mentioned.
ADDED: In retrospect, I am now unsure whether this is what was asked for. Sorryparticular, in case it should not be. (Zhich in viewlater parts various of a comment, appearing while I added the disclaimer, seemsHasse's paper related to be the casethis are discussed.)