ath10k: prevent starting monitor without a vdev

This fixes issue with interface bridging.

Linux bridging sets promiscuous mode for all
interfaces that are in a bridge. This translates
to configure_filter() being called in a mac80211
driver.

Before the patch operational interface would be
started and upped again when promiscuous mode was
enabled causing all sorts of strange issues:

 * no HTT RX happening (i.e. no traffic)
 * FW crash upon driver reload/unload

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