ath5k: reorder base.c to remove fwd decls

This change reorganizes the main ath5k file in order to re-group
related functions and remove most of the forward declarations
(from 61 down to 3).  This is, unfortunately, a lot of churn, but
there should be no functional changes.
Signed-off-by: NBob Copeland <me@bobcopeland.com>
Signed-off-by: NBruno Randolf <br1@einfach.org>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>