Specify the NSS index for 802.11ac multicast rates.
The value 0 indicates that no NSS index is specified.
No NSS index is specified for 802.11ac multicast rates by default.
The specified NSS index for 802.11ac multicast rates must be no less
than the specified maximum NSS index for 802.11ac mandatory rates.
Before configuring the NSS index for 802.11ac multicast rates, the maximum
NSS index for 802.11ac mandatory rates must be configured.
Besides, it must be configured together with the node below.