ath10k: don't remove peer that doesn't exist

If peer creation failed during offchannel Tx the
driver attempted to delete the peer nonetheless.
This caused the ar->num_peers counter to be
incorrectly decremented. This subsequently could
cause the counter to drop below 0 and also
eventually lead to firmware crash because host
would think there are less peer entries created in
firmware then there really were.

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