ath9k_hw: merge codepaths that access the cycle counter registers

The cycle counters are used by ANI to determine the amount of time that the
radio spent not receiving or transmitting. They're also used for debugging
purposes if the baseband watchdog on AR9003 detects a lockup.
In the future, we want to use these counters to determine the medium utilization
and export this information via survey. For that, we need to make sure that
the counter is only accessed from one place, which also ensures that
wraparounds won't occur at inconvenient points in time.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>