wireless: fix Atheros drivers compilation

Bug introduced in commit:
wireless: allow Atheros card to not depend on ath.ko

Commit in question changed CONFIG_ATH_COMMON to CONFIG_ATH_CARDS as
"Atheros card" indication in drivers/net/wireless/ath/Kconfig but it
is used also by drivers/net/wireless/Makefile

If there are only Atheros cards that do not require ATH_COMMON, whole
Makefile for Atheros cards was not executed; and as result, driver
won't compile in this case.

Change in CONFIG_ option name should be reflected in the
drivers/net/wireless/Makefile

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