ath9k: Modify ATH9K_BTCOEX_SUPPORT

ATH9K_BTCOEX_SUPPORT is now used by both ath9k and ath9k_htc
to enable BT coexistence. Fix Kbuild to allow this.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>