ath10k: support for multicast 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.

Tested on QCA9984 with firmware ver 10.4-3.6-00104

Signed-off-by: Pradeep Kumar Chitrapu <pradeepc@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>