ath10k: fix device teardown

This fixes interrupt-related issue when no
interfaces were running thus the device was
considered powered down.

The power_down() function isn't really powering
down the device. It simply assumed it won't
interrupt. This wasn't true in some cases and
could lead to paging failures upon FW indication
interrupt (i.e. FW crash) because some structures
aren't allocated in that device state.

One reason for that was that ar_pci->started
wasn't reset. The other is interrupts should've
been masked when teardown starts.

The patch reorganized interrupt setup and makes
sure ar_pci->started is reset accordingly.

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