ath5k: Cleanups v2 + add kerneldoc on all hw functions

No functional changes

Add kernel doc for all ath5k_hw_* functions and strcucts. Also do some cleanup,
rename ath5k_hw_init_beacon to ath5k_hw_init_beacon_timers, remove an unused
variable from ath5k_hw_pcu_init and a few obsolete macros, mostly related to XR.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>