ath5k: Enable AR2315 chipset recognition

Enable recognition of AR2315 chipsets in ath5k driver.

Reported-by: Simon Morgenthaler <s.morgenthaler@students.unibe.ch>
Signed-off-by: Wojciech Dubowik <Wojciech.Dubowik@neratec.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>