ath5k: Wakeup fixes

* Don't put chip to full sleep because there are problems during
   wakeup. Instead hold MAC/Baseband on warm reset state via a new
   function ath5k_hw_on_hold.

 * Minor cleanups

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Tested-by: Ben Greear <greearb@candelatech.com>
Tested-by: Johannes Stezenbach <js@sig21.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>