[BUGFIX] Use correct getter for FormEngine input name in Slider

Slider still fetches on name in JavaScript instead of the new
data attribute.

Resolves: #70195
Releases: master
Change-Id: I0c205e547c33f3fe3c3c8a4df1bbb44ae154be56
Reviewed-on: http://review.typo3.org/43636


Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>