ath9k: Fix Kconfig for ATH9K_HTC

Ath9k consists of 2 different sub-modules: ATH9K and ATH9K_HTC.
Both uses common Atheros code from ath.ko and need ATH_COMMON.

However, while ATH9K selects ATH_COMMON, ATH9K_HTC does not.
As result, if ATH9K_HTC is the only Atheros card selected, compilation fails with
unresolved symbols.

This patch moves ATH_COMMON selection to the common part for both
ATH9K and ATH9K_HTC

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