luci-base: fix button handling for named sections

If more than one named section is added to the page, it is currently the
case, that the first button is always switched on or off during input
validation of the uci section name. This is because the usage of the
'document.querySelector' function is to imprecise. Changing the search
start to the element to be created, fixes this.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>