ath9k_hw: Fix PAPRD training

The PAPRD training control registers have to be
programmed with values that depend on the chip. This patch
ensures that the correct values are chosen for the chip
in use.

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