luci-mod-system: flash.js: fix passing sysupgrade opts

Instead of binding the checked state to the button handler function,
bind the underlying checkboxes instead to observe the actual user
choice instead of the initial value.

This fixes forcing sysupgrade or deselecting keep settings.

Reported-by: Hannu Nyman <hannu.nyman@iki.fi>
Signed-off-by: Jo-Philipp Wich <jo@mein.io>