xprofile: fix undefined variable notice when changing a profile field's visibility setting.

r7617 introduced a bug where the wrong variable was being referenced, causing a PHP notice, and breaking the edit form. Props graham-washbrook, fixes #5286

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7655 cdf35c40-ae34-48e0-9cc9-0c9da1808c22