ath9k: Fix BTCOEX timer triggering comparision

Its more correct to convert btcoex_period to 'us' while
comparing with btcoex_no_stomp which is in 'us'.
Did not find any functionality issues being fixed,
as the generic hardware timer triggers are usually
refreshed with the newer duty cycle.

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