udlfb: Search config descriptor if not at device level

For at least one DisplayLink device, the vendor-specific information can
be found in the config descriptor instead of as a separate, device-level
descriptor.  This patch searches the current interface (of the current
config descriptor) for the DL vendor-specific descriptor.

Signed-off-by: Andrew Kephart <akephart@akephart.org>
Signed-off-by: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>