ath10k: advertise p2p dev support

Firmware doesn't allow precise tx rate control so
P2P wasn't entirely spec compliant (it was using
CCK rates in some cases).

The only way to make sure firmware doesn't use CCK
rates is to have a vdev with P2P subtype used for
scanning and tx. This can be done via a special
dedicated P2P device interface support.

This also removes the ancient hack from ath10k in
favor of p2pdev.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>