ath9k_htc: Cancel BTCOEX related work before disabling BTCOEX

Before disabling BTCOEX in the h/w cancel all BTCOEX related
works. This is similar to the commit in ath9k(c32cdbd8

)
ath9k: Stop the BTCOEX timers before disabling BTCOEX

Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>