wifi-scripts: fix mcast_rate type (number, not array)

wifi-scripts: fix mcast_rate type (number, not array)

The multicast rate (mcast_rate) is defined as a single numeric value,
not an array. The ucode scripts already expect a single number, and
the OpenWrt documentation specifies this option as an integer [0].

Adjust the wireless schema accordingly. This also fixes the wdev.uc
call path for the iw fallback mesh setup.

[0] - https://openwrt.org/docs/guide-user/network/wifi/basic

Link: https://github.com/openwrt/openwrt/pull/21291
Signed-off-by: Nick Hainke <vincent@systemli.org>