drm/udl: Treat vendor descriptor as u8

The vendor descriptor is an array of unsigned bytes. It is raw data
that is not to be modified. Declare it as 'const u8'.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://lore.kernel.org/r/20250410105948.25463-8-tzimmermann@suse.de