[BUGFIX] Pass $field to BackendUtility::getFlexFormDS

This bugfix just passes an already existing argument
along to the hook method, making the $field variable
available there.

Change-Id: I5337490920a96a8db0af8f8ecc01e967dba0c9ca
Fixes: #39527
Releases: 6.2, 6.1, 6.0, 4.7, 4.5
Reviewed-on: https://review.typo3.org/20919
Reviewed-by: Dmitry Dulepov
Tested-by: Dmitry Dulepov
Reviewed-by: Philipp Gampe
Tested-by: Philipp Gampe
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters