ath9k_htc: Revamp CONF_IDLE handling

This patch revamps IDLE power save handling in the driver.
Two separate functions (radio enable/disable) are introduced,
because the semantics of radio handling is just not the same
as the start()/stop() callbacks. For example, the HW must not
be disabled, instead, the PHY has to be disabled in radio_disable().
Also, the HW has to be reset properly in radio enable/disable and
certain registers have to be programmed only once, in the
start() callback.

The radio_enable() routine doesn't need the PS wrappers since
we set the HW power mode to AWAKE anyway before calling it.

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