ath9k: Clean up RX processing a bit

This makes use of the local fc variable in bit more places and uses a
common helper macro. The part of RX process that delivers skb's to
mac80211 is moved to a separate function in preparation for future
changes that will need to do this from two places. The modifications
here should not result in any functional changes.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>