ath9k: Add IEEE80211_HW_RX_INCLUDES_FCS to hw flags

Don't trim the FCS before passing the frame to mac80211,
move TSF_TO_TU to core.h and delete more unused macros.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>