USB: isp1760: use a specific PLX bridge instead of any bdridge

commit 6c0735687d37e25a65866823881bcbf39a6a023f upstream.

this driver can't handle (of course) any brdige class devices. So we
now are just active on one specific bridge which should be only the
isp1761 chip behind a PLX bridge.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Tested-by: Karl Bongers <kblists08@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>