drm/fsl-dcu: Fix return type of fsldcudrmconnectormode_valid()
This is an automated ID intended to aid in discovery of potential security vulnerabilities. The actual impact and attack plausibility have not yet been proven. This ID is fixed in Linux Kernel version v4.9.337 by commit 1cca6ac0a7982e6cd3c197d3049dd6571cf07292. For more details please see the references link.