ath9k: fix signal strength reporting issues

On A-MPDU frames, the hardware only reports valid signal strength data for
the last subframe. The driver also mangled rx_stats->rs_rssi using the
ATH_EP_RND macro in a way that may make sense for ANI, but definitely
not for reporting to mac80211.
This patch changes the code to calculate the signal strength from the rssi
directly instead of taking the average value, and flag everything but
the last subframe in an A-MPDU to tell mac80211 to ignore the signal strength
entirely, fixing signal strength fluctuation issues reported by various
users.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>