ath9k: move ath9k_rx_skb_postprocess to common.c

and rename it to ath9k_cmn_rx_skb_postprocess. We will use it
on ath9k_htc.

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