ath5k: correct ath5k signal quality

This adjusts ath5k's signal quality reporting taking into
consideration received beacons at an RSSI of 35 should indicate
you can use of 54 Mbps on a single stream card (all ath5k cards)
reliably.

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