xProfile: fix datebox field validation.

If a validation error occurred when saving a profile field(s), the
datebox field type wasn’t restoring the user-submitted value after the
screen was reloaded. This was a regression introduced in BuddyPress 2.0
with the Profile Field Types rewrite.

Fixes #5690 (trunk), props esroyo for initial patch

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8652 cdf35c40-ae34-48e0-9cc9-0c9da1808c22