ath9k: move rssi processing into a helper

This moves all the RX processing of RSSI into a helper,
ath_rx_prepare(). ath_rx_prepare() should now be really
easy to read and follow.

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