ath9k: move ath9k_process_rssi to common.c

we can reuse this fucntion on ath9k_htc.
Now we will need to use common version last_rssi, so switch
it too.

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>