[BUGFIX] Respect global renderReadonly in Tceforms for tree

The global renderReadonly status in Tceforms doesn't affect a
rendered tree because the check was forgotten when #25888 was
implemented. An additional check is added.

Resolves: #59849
Releases: 6.3, 6.2
Change-Id: Ic89ab2f39636dbbb7dc37941af8fe7be085fefcf
Reviewed-on: http://review.typo3.org/31815
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>