Revert "luci-mod-network: ra_preference option uses '' instead of "medium""

This reverts commit 6bcdc18d65.

Since the bug where a write operation is executed when the input
matches the default has been fixed in form.js, this workaround
is no longer needed and is being reverted.

Signed-off-by: Andy Chiang <AndyChiang_git@outlook.com>

modules/luci-mod-network/htdocs/luci-static/resources/view/network/interfaces.js

@@ -897,8 +897,9 @@ return view.extend({
 
 					so = ss.taboption('ipv6-ra', form.ListValue, 'ra_preference', _('Router Priority'),
 						_('A tie-breaker for clients and their routes when multiple routers exist on the same network.'));
-					so.value('', _('Medium'));
+					so.default = 'medium';
 					so.value('low', _('Low'));
+					so.value('medium', _('Medium'));
 					so.value('high', _('High'));
 					so.depends('ra', 'server');