ath9k: Stop the BTCOEX timers before disabling BTCOEX

Its safe to stop the BTCOEX timers 'period_timer' and
'no_stomp_timer' before disabling BTCOEX. These timers
can call ath9k_hw_btcoex_enable (or) change the BT
stomp type if they seem to be running after we had
called ath9k_hw_btcoex_disable, which is obviously
not correct.

Cc: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Cc: Bala Shanmugam <bkamatch@qca.qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>