ath10k: clean up num_peers locking

The var was supposed to be protected by data_lock
but it wasn't so in all instances. It's actually
not necessary to have a spinlock protected
num_peers so drop it.

All instances of num_peers are already within
conf_mutex sections so use that.

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