ath9k_hw: Add HW cap for PAPRD

Add a HW capability to indicate whether PAPRD is enabled
for the card, since PAPRD could be enabled in the EEPROM, but
disabled in the driver. This makes things clearer.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>