wifi-scripts: wdev.uc: fix mesh mode frequency handling

wifi-scripts: wdev.uc: fix mesh mode frequency handling

Mesh mode interface creation fails when the freq parameter is empty or
undefined. Unlike adhoc mode which checks if freq exists before using it,
mesh mode blindly constructs the iw command with freq parameter, resulting
in invalid syntax like:

  iw dev mesh0 mesh join ssid freq  NOHT

This causes the mesh interface to be created without joining the mesh
network, leaving it in a DOWN state with no channel assigned.

Fix by adding freq validation check similar to adhoc mode.

Tested on two routers in parallel as mesh peers:
- Xiaomi AX3000T (MediaTek MT7981)
- OpenWrt One (MediaTek MT7981)
- OpenWrt 6.6.119, 802.11s mesh on 5GHz (Channel 36, HE80)

Signed-off-by: Valent Turkovic <valent@meshpointone.com>
Link: https://github.com/openwrt/openwrt/pull/21373
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>