ath9k: Initialize BTCOEX scheme using a helper

Setting up the required scheme can be done as part of the
BTCOEX initialization path and it doesn't belong in
ath9k_hw_fill_cap_info() anyway.

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