ath9k: introduce ATH9K_{PCI,AHB} config options

Currently ath9k only available in menuconfig if PCI bus
support is enabled. However the driver is required for
the built-in wireless MACs of the Atheros AR9130/AR9132
SoCs. These SoCs have no PCI controller, the wireless
MAC is connected to the AHB bus on them.

Introduce separated config options for the supported
buses, in order to allow building of ath9h without PCI
bus support.

As a bonus, this patch removes the cross-reference of
the ATHEROS_AR71XX option which is not present in the
kernel.

Cc: Luis R. Rodriguez <lrodriguez@atheros.com>
Cc: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>