drm/udl: Sync pending URBs at suspend / disconnect

We need to wait for finishing to process the all URBs after disabling
the pipe; otherwise pending URBs may stray at suspend/resume, leading
to a possible memory corruption in a worst case.

Tested-by: Thomas Zimmermann <tzimmermann@suse.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220804075826.27036-3-tiwai@suse.de