ath9k: differentiate quality reporting between legacy and HT configurations

We were not differentiating quality between legacy and HT
configurations. We change this to consider the differences.

New theory for reporting quality:

At a hardware RSSI of 45 you will be able to use MCS 7  reliably.
At a hardware RSSI of 45 you will be able to use MCS 15 reliably.
At a hardware RSSI of 35 you should be able use 54 Mbps reliably.

MCS 7  is the highets MCS index usable by a 1-stream device.
MCS 15 is the highest MCS index usable by a 2-stream device.

All ath9k devices are either 1-stream or 2-stream.

How many bars you see is derived from the qual reporting.

A more elaborate scheme can be used here but it requires tables
of SNR/throughput for each possible mode used. For the MCS table
you can refer to the wireless wiki:

http://wireless.kernel.org/en/developers/Documentation/ieee80211/802.11n

This should fix this bug report:

http://bugzilla.kernel.org/show_bug.cgi?id=13537



Cc: Janath.Peiris@atheros.com
Cc: Matt.Smith@atheros.com
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>