luci-admin-full: add further wifi options

This commit adds option to disable scan for 40mhz channel, permit to tweak
beacon interval and other advanced settings.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>
[fix whitespace, add range constraint to dtim_period, add dtim_period
 to local vars, reword commit message]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>