[BUGFIX] selected = 1 doesn't work in FormContentObject

Wrong variable is used to set selected correctly.

Change-Id: I90623339c4d13eb07d7e5ef87e1b4e3aa8cf147e
Resolves: #43893
Releases: 6.2, 6.1, 6.0
Reviewed-on: https://review.typo3.org/20282
Reviewed-by: uwe trotzek
Tested-by: uwe trotzek
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
(cherry picked from commit 8d30ece2f0dfc0306c22a63d9e57a75e32d1365e)
Reviewed-on: https://review.typo3.org/22751