ath5k: Add a function to read chipset's MAC revision

Add bus dependent revision read function which is used to
determine chipset's MAC before hardware is initialized.

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>