ath9k_hw: move the cycle counter tracking to ath

Instead of keeping track of wraparound, clear the counters on every
access and keep separate deltas for ANI and later survey use.
Also moves the function for calculating the 'listen time' for ANI

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>