ath9k: Fix bug in configuring hw timer

Hw next tigger time is configured as current_tsf + (timer_period * 10) which
is wrong, it should be current_tsf + timer_period. The wrong hw timer configuration
would cause btcoex related issues.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>