ath9k: Nuke struct ath9k_node_stats

Other than ns_avgbrssi (Average beacon rssi) no data of
ath9k_node_stats is being used anywhere. Nuke this structure
and move its only useful member to ar5416Anistate. Also cleanup
this redundant data in ath_softc.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>