ath5k: Add AHB bus support.

AHB specific functions are now in ahb.c file. AHB bus is
compiled in when CONFIG_ATHEROS_AR231X is set in kernel.
All other platforms will use PCI bus.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Wojciech Dubowik <Wojciech.Dubowik@neratec.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>