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: openwrt/openwrt#21291
(cherry picked from commit a6eb3d9)
Link: openwrt/openwrt#21301
Signed-off-by: Nick Hainke <[email protected]>

Author: PolynomialDivision

package/network/config/wifi-scripts/files-ucode/usr/share/schema/wireless.wifi-iface.json

@@ -605,11 +605,8 @@
 			"type": "boolean"
 		},
 		"mcast_rate": {
-			"description": "Allowed multicast rates",
-			"type": "array",
-			"items": {
-				"type": "number"
-			}
+			"description": "Allowed multicast rate",
+			"type": "number"
 		},
 		"mesh_auto_open_plinks": {
 			"type": "boolean"