ath10k: Calculate correct peer PHY mode for VHT

The peer PHY mode for 11ac operation needs to be determined
properly based on the channel bandwidth being used. Fix
this so that the proper mode is given to the firmware.

kvalo: earlier we used 11na-ht20 in STA mode for 11ac AP peer, this
patch changes that to 11ac-vht80. I didn't notice any change in
throughput in my tests, but nevertheless it's the right thing
to do.

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