luci-base: ui.js: support div based table markup in UITable.update()

Constructing UITable instances from existing, div based markup is
supported but the UITable.update() implementation did not account
for that, leading to defunct data updates on tables built from div
based markup.

Fix this issue by extending UITable.update() to consider a div based
table structure as well, like we do in UITable.initFromMarkup() already.

Fixes: #5713
Signed-off-by: Jo-Philipp Wich <jo@mein.io>