usb/isp1760: Fix possible unlink problems

Use skip map to avoid spurious interrupts from unlinked transfers.
Also changes to urb_dequeue() and endpoint_disable() to avoid
release of spinlock in uncertain state.

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