ath10k: fix incorrect multicast/broadcast rate setting

Invalid rate code is sent to firmware when multicast rate value of 0 is
sent to driver indicating disabled case, causing broken mesh path.
so fix that.

Tested on QCA9984 with firmware 10.4-3.6.1-00827

Sven tested on IPQ4019 with 10.4-3.5.3-00057 and QCA9888 with 10.4-3.5.3-00053
(ath10k-firmware) and 10.4-3.6-00140 (linux-firmware 2018-12-16-211de167).

Fixes: cd93b83a ("ath10k: support for multicast rate control")
Co-developed-by: Zhi Chen <zhichen@codeaurora.org>
Signed-off-by: Zhi Chen <zhichen@codeaurora.org>
Signed-off-by: Pradeep Kumar Chitrapu <pradeepc@codeaurora.org>
Tested-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>