ath10k: fix peer limit enforcement

Firmware peer entries are involved in internal
firmware vdev structures. This was not accounted
for and could lead firmware to crash due to asking
it to do more than it could.

Fixes: 039a0051 ("ath10k: allocate fw resources for iface combinations")
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>