[BUGFIX] Do not drop selection from "singlebox" select

By setting the new value after validation via attribute, we avoid
triggering a new "change" event which in turn causes a validation
again.

Resolves: #69490
Releases: master
Change-Id: I679da7ad7b71048a10a07de1f2d290bfde0dde8b
Reviewed-on: http://review.typo3.org/43343


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Thorsten Bringewatt <t.bringewatt@mittwald.de>
Tested-by: Thorsten Bringewatt <t.bringewatt@mittwald.de>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>
Tested-by: Frank Nägler <frank.naegler@typo3.org>