ath9k: Add a macro to identify PCOEM chips

This can be used if we need to apply register settings
for all PCOEM solutions (in the AR9003 family).

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>