ath10k: fix station count enforcement

The number of peers isn't directly translatable to
the number of stations because ath10k needs to
reserve a few extra peers for special cases like
multi-vif concurrency.

The previous limit was 126 and 15 stations in AP
mode for 10.x and main firmware branches
respectively. The limit is now 128 and 16 which
was the original intention.

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