ath5k: Introduce ath5k_init_softc function as in ath9k

Split pci initialization into hardware specific
functions and softc structure initialization.
Make function naming similar to ones ath9k.
Introduce ath_bus_opts in ath5k for later
AHB bus integration.

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>