drm/udl: Suppress error print for -EPROTO at URB completion

The driver may receive -EPROTO at the URB completion when the device
gets disconnected, and it's a normal situation.  Suppress the error
print for that, too.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220908095115.23396-6-tiwai@suse.de