drm/udl: Return error if vendor descriptor is too short

There need to be least 5 bytes in the vendor descriptor. Return
an error otherwise. Also change the branching to early-out on
the error. Adjust indention of the rest of the parser function.

The original length test expected more than 5 bytes in the vendor
descriptor. As a descriptor with no key-value pairs has exactly 5
bytes and is still valid, change the test to accept this case as
well.

v2
- clarify changes to length test in commit description (Patrik)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://lore.kernel.org/r/20250410105948.25463-7-tzimmermann@suse.de