wireless: allow Atheros card to not depend on ath.ko

There are 2 different things:
- sub-menu for "Atheros Wireless cards" family
- module ath.ko with common Atheros code

Until now, they both used to depend on the same Kconfig variable ATH_COMMON.
Thus, being "Atheros card" and "depending on ath.ko" was the same.

To allow module to belong to the
"Atheros Wireless cards" family but not use ath.ko,
2 conditions above need to be separated.

So, this patch introduce new Kconfig variable ATH_CARDS for belonging
to the "Atheros Wireless Cards" family; while ATH_COMMON becomes hidden
variable to express dependency on common Atheros code in ath.ko. Modules
that depend on this common code now express it by setting ATH_COMMON.

Right now, ath6kl do not depend on common code and thus do not set ATH_COMMON.

Signed-off-by: Vladimir Kondratiev <qca_vkondrat@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>