udlfb: fix issues found with Sparse static analysis

Add __user casting, a missing copy_from_user, and proper boolean

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Signed-off-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>