ath9k: Fix airtime calculation for quarter/half channels

The bitrate value for airtime calculation is specified for
full rates. We need to divide it for 5 and 10MHz channels to
get correct result.

Signed-off-by: Wojciech Dubowik <Wojciech.Dubowik@neratec.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>