ath10k: remove ar_pci->started

There are basically no more uses for
ar_pci->started. It is also perfectly safe to call
hif_stop without hif_start now.

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