ath10k: decouple pci start/stop logic

Split logic that prepares the device for BMI
phase/cleans up related resources.

This is necessary for ath10k to be able to restart
hw on the fly without reloading the module.

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