luci-mod-admin-full: fixes for usbport LED triggers

The previous commit erroneously used "ports" instead of "port" as name for the
option widget, causing wrong uci values to be written.

Also work around some cbi idiosyncrasies regarding MultiValue widgets which
prevented rendering the correct initial selection state.

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