ath10k: reset device upon stopping/power down

This should make sure the device won't issue any
interrupts nor access any memory after the driver
is stopped/freed thus avoid memory corruption in
some cases.

Reported-By: Ben Greear <greearb@candelatech.com>
Reported-By: Janusz Dziedzic <janusz.dziedzic@tieto.com>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>