ath9k: Fix wifi disconnection when collocated bt scan is active

As all bt packets are priority traffic during bt scan, wifi
will get disconnected when bt scan lasts for few seconds. Fix
this by allocating 10% of bt period time (4.5ms) to wifi fully.

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