ath9k: Fix NF CCA limits for AR9287 and AR9227

ath9k: Fix NF CCA limits for AR9287 and AR9227

The FreeBSD driver [0] uses the same 2G values as for the AR9280 chips.
Using the same values in ath9k results in much better throughput for me.

Before this patch I had a huge amount of packet loss (sometimes up to
40%) and the max transfer speed was somewhere around 5Mbit/s. With this
patch applied I have zero packet loss and ten times the throughput.
My device uses a AR9227 which is the PCI variant of the AR9287.

[0] http://bxr.su/FreeBSD/sys/dev/ath/ath_hal/ar9002/ar9287.h

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>