luci-mod-status: fix channel_analysis ReferenceError: diff is not defined

In refactoring the code there was an error and the sort function wasn't
correctly reworked with diff not correctly dropped.

Drop that and correctly sort the WiFi ap if the channel is different.

Fixes: 75dcb097

 ("luci-mod-status: improve channel_analysis page")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>