usb/isp1760: Fix crash when unplugging bug

This fixes a problem with my previous patch series where there's a great
risk that the kernel will crash when unplugging interrupt devices from
the USB port. These lines must have got missing when I rebased the
patches from the older kernel I was working with to 2.6.37 and 2.6-next:

This fixes a bug where the kernel may crash if you unplug a USB device
that has active interrupt transfers.

Signed-off-by: Arvid Brodin <arvid.brodin@enea.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>