ath9k: Prune ath9k_init_device

Instead of cluttering ath9k_init_device() with 'ath_softc'
specific initialization code, use ath9k_init_softc() to
setup driver-specific work routines, timers etc.

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