ath9k: correct signal quality report

This adjusts ath9k's signal quality reporting taking into
consideration received beacons at an RSSI of 45 should indicate
you can use of MCS 15 rate (144 Mbps) reliably on dual stream card.
Keep in mind mac80211 does not yet have aggregation fixed but this
should still provide more accurate results.

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