ath10k: fix ldpc param for fixed rate

ldpc is not configured for fixed rates. This blocks auto rate vs fixed
rate performance comparison. Since firmware is considering ldpc vdev
param for fixed rate selection, it has to be configured to enable ldpc
for fixed rates.

Signed-off-by: Rajkumar Manoharan <rmanohar@qti.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>