drm/udl: Handle errors from usb_get_descriptor()

Reading the vendor descriptor from the udl device can fail with
an error, which the current code fails to capture. Store the return
value in an integer and test for the error. Abort parsing on errors
or treat the value as length on success.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://lore.kernel.org/r/20250410105948.25463-6-tzimmermann@suse.de