Signup Wizard: Multiple Campuses are selected on the Campus Picker, causing issues when saving the connection requests

[tcavaletto]

https://github.com/secc/RockPlugins/blob/master/Plugins/org.secc.Connection/org_secc/Connection/VolunteerSignupFormConnections.ascx.cs#L488 The SetValue function does not un-select other options, so this ends up with two options selected if a campus guid/id is passed in through the page parameters. This causes issues with getting the correct SelectedCampusId when saving the connection request.