ath9k_htc: fix signal strength reporting issues

Backport of r35749

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@35751 3c298f89-4303-0410-b956-a3cf2f4a3e73
ath9k: fix rssi issues

Backport of r35750

git-svn-id: svn://svn.openwrt.org/openwrt/branches/attitude_adjustment@35752 3c298f89-4303-0410-b956-a3cf2f4a3e73