ath5k: report PHY error frames only for chips which need it

Only report PHY error frames for ANI on chipsets which do not have PHY error
counters in hardware.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>