mac80211: ath10k: support for management rate control

mac80211: ath10k: support for management rate control

Issues a wmi command to firmware when multicast rate change is received with the
new BSS_CHANGED_MCAST_RATE flag.  Also fixes the incorrect fixed_rate setting
for CCK rates which got introduced with addition of ath10k_rates_rev2 enum.

By default the firmware uses 1Mbps and 6Mbps rate for management packets
in 2G and 5G bands respectively. But when the user selects different
basic rates from the userspace, we need to send the management
packets at the lowest basic rate selected by the user.

Signed-off-by: Sven Eckelmann <sven@narfation.org>