ath10k: skip fw stats debugfs interface if device is down

If the device is not running then there may be no
FW at all to send the query to. If the FW is
already there it might still trigger a crash if
the command is sent before the device is fully
initialized.

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