The code that manges the synchronous control/bulk calls has
been a mess for ages.  This patch rewrites it using:

 - "struct completion" instead of a usb-internal clone therof,
 - prepare_to_wait()/finish_wait() instead of the tangled
   mess it now uses (or a new wait_event_timeout call, as in
   previous versions of this patch).

It's a net code shrink and simplification.