ath9k: Trim struct ath_softc

Add sc_rxflush and sc_noreset as bitfields to sc_flags.
Remove a few variables and function prototypes that are unused.

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