[BUGFIX] Numeric check for upper/lower bound of flexform-values

The range-check in Flexforms uses the same code as for normal
tables. Normally an integer field would be of type integer.
But in Flexforms the value is a string as is the value in DS.

Change-Id: I3512a961780a4bef91e04f61b8b9547f43f28f3e
Fixes: #15771
Releases: 4.5, 4.6, 4.7, 6.0, 6.1
Reviewed-on: https://review.typo3.org/9022
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer