ath10k/ath10k-ct: support multicast and management rate control

Drivers with software rate control can directly use the selected multicast
rate for multicast/broadcast frames and the minimal basic rate for
management frames. But drivers with offloaded rate control algorithms must
be informed about such upper layer decisions to configure the
hardware/firmware.

A new BSS_CHANGED_MCAST_RATE is introduced in mac80211 to automatically
inform all drivers. ath10k can detect this event and forward it via WMI to
the driver. The already existing BSS_CHANGED_BASIC_RATES can be used to
select the management rate.

Without the WMI commands, a low rate (not necessarily one from the basic
rates) is used for bcast/mcast/management frames. This means that the
/etc/config/wireless settings basic_rate and mcast_rate would have no
effect on the rates selected by this driver for the mentioned frames.