ath10k: refactor monitor code

It was possible to create/delete/start/stop
monitor vdev from a few places that were not
exclusively protected against each other. This
resulted in monitor vdev being stopped/removed by
one call origin while another one was expecting it
to continue running.

For example if CAC was started and interface's
promiscuous mode was toggled monitor vdev was
removed from the driver meaning no radar would be
detected. In additional a warning would be printed
upon CAC completion complaining it tried to stop
non-running monitor vdev.

The patch simplifies monitor code by removing
IEEE80211_HW_WANT_MONITOR_VIF (which wasn't really
ever needed) and improves state tracking. It also
unifies prints.

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