ath5k: minor cleanup

Move an extern variable declaration to a header file.  Prefix all
functions with "ath5k_".  Make ath5k_intr() static.  Remove unneeded
forward declarations.  Remove pointless "extern" in a function
declaration.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>