ath9k: Remove enabling btcoex from stomp type change

This patch removes btcoex_enable from stomp type change and let
it be called from callee functions that makes the code can be
reusable for MCI changes.

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