ath5k: clean up base.h and its use

Remove unnecessary includes from base.h.  Add includes to other files as
necessary.  Don't include base.h unless needed.

Move declarations for functions in base.c from ath5k.h to base.h.

Use a better named define to protect base.h against double inclusion.

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