ath9k: Cleanup init/deinit routines

The device initialization and termination functions
were messy and convoluted. Introduce helper functions
to clarify init_softc() and simplify things in general.

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