ath9k: Fix build failure

Make sure that CONFIG_ATH9K_BTCOEX_SUPPORT is used for
the WLAN/BT RX diversity hooks.

Reported by the kernel build testing backend.

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