ath9k_hw: fix PHY counter overflow handling in ANI v1

PHY counter overflows need to be checked for the old ANI version,
because of its use of interrupt based counter overflow reports when
the counters exceed the configured thresholds.

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