ath9k: Fix bug in retrieving average beacon rssi

Currently the beacon rssi that LPF gives is divided and rounded
up by ATH_RSSI_EP_MULTIPLIER twice. This will leave the incorrect rssi
in ANI. Having correct rssi in ANI fixes the connection stability at
< 30dB rssi range. This patch removes the unncessary computation of average
rssi over already valid average rssi. Also removes the redundant macros to
find average rssi.

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