modules/admin-full: fix wifi channel selection on multiple STA networks

Besides several AP networks, I have configured two STA networks on my
openwrt box - both on the same radio and thus on the same channel.
This was done via LuCI.

However after both STA networks were set up, I am unable to edit the
channel on neither network. When editing the one STA network, LuCI
tells me that the channel is locked by the other STA network. Same for
the other STA network.

Looks like a bug to me, so I made a patch.

Signed-off-by: Stephan Günther <steph.guenther@googlemail.com>