ath9k: Fix MCI scheme initialization

Commit "ath9k_hw: remove ATH_BTCOEX_CFG_MCI" removed
MCI as a separate coex scheme, but we need it to
avoid fiddling with GPIO registers during
ath9k_init_btcoex() that are meant only for 3-wire
cards.

Cc: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>