drm/udl: Sync pending URBs at the end of suspend

It's better to perform the sync at the very last of the suspend
instead of the pipe-disable function, so that we can catch all pending
URBs (if any).

While we're at it, drop the error code from udl_sync_pending_urb()
since we basically ignore it; instead, give a clear error message
indicating a problem.

Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220908095115.23396-13-tiwai@suse.de