usb/isp1760: Improve urb queueing, get rid of BUG():s in normal code paths

This patch replaces the code that handles qtds. Intead of directly allocating
chip mem and chip slot, enqueue the transfer in a list of queue heads. Use
a centralized function enqueue_qtds() to prioritize and enqueue transfers.

This removes all of the interrupt context BUG() calls when out of chip
mem or transfer slots. It also makes it possible to efficiently use the
dual-port mem on the chip for double-buffered transfers, which improve
transfer times to/from/between usb sticks by about 40 % on my HW.

With this patch it should also be possible to handle qtd scheduling outside
of the interrupt handler, for significantly improved kernel latency. I have
not implemented this since there are some locking issues which I haven't
had time to look at.

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