luci-base: validation.js: fix "unique" validator

Previous refactoring renamed the "data-type" attribute of widget markup
containers to "data-widget", breaking the "unique" validator as it relies
on it to lookup options.

Fixes: #3341
Fixes: 13e9e3e9

 ("treewide: fix "Unhandled token" errors with Lua CBI maps")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>