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

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>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>